Fritz's height
Fritz's height is variously reported as 5'2" (157 cm) and 5'7" (170 cm) in the (English language) sources I've read, with little to no middle ground.
The confusion seems to arise from a difference between French units and English units. Voltaire, in his 1752 pamphlet, described Fritz as "cinq pieds deux pouces." A 1775 English translation rendered this "five feet two inches high."
Unfortunately, in the 18th century, "five feet two inches" in French was the equivalent of about "five feet six inches" in English. This is the same thing that happened to Napoleon, who was evidently the same height as Fritz, and got a reputation for being extremely short due to the failure of the English to convert, and possibly also due to his spending a lot of time around tall guards (which was probably also a factor in Fritz's case, though not as much as his father!).
In Fritz's case, the fact that he lived into his mid seventies, doubtless suffered spinal compression like most adults who live that long (it's a sheer factor of gravity), and was described as having his head bent nearly to his shoulder from arthritis in the last year of his life, i.e., the fact that he was at his shortest when he was also at his most famous, may have contributed to the descriptions of him as 5'2". But we have textual evidence that there was a failure to convert to English inches even when he was 40.
ETA: We now have the testimony of Schöning, one of Fritz's chamber hussars, that Frederick II was about 5 feet 5 inches tall. Give or take an inch, and that one is writing after Fritz died and one in 1752, he probably lost an inch or two in 34 years, from spinal compression.
Fritz's weight
Once upon a time,
selenak reported:
Having spent several days in Saxony this week, I also visited Königstein Castle, where they have the very special big scales where August the Strong got himself and his guests of honor weighed. And thus I can report Fritz' weight in the year 1728, fully clothed, was 64 kilo.
At the time I only knew the two radically different heights, but now that we know Fritz was approximately 1.67 m (5'6"), and the lower height is due to French/English unit confusion, we have his height and weight at age 16! 64 kilos is 140 lbs, which is not underweight for that height.
FW's weight
FW, I've seen reported as 5'2" (157 cm), but that may be a misunderstanding similar to Fritz's, because I found an article in this volume (p. 38) that reports him as 1.65m.
FW's weight
According to the record at the Königstein scales, as reported by
selenak, FW was 108 kilos in 1728.
According to the volume cited above that records FW's height as 1.65 m, his weight was 2 1/2 "Zentner" at the age of 35 (so 1723). With one Zentner being about 50 kilos, that's supposed to be 125 kilos (275 lb), according to Wikipedia (though of course units vary by time and place!).
Most likely adult heights and weights from 1728:
FW: 165 cm (5'5"), 125 kilos (275 lbs)
Fritz: 167 cm (5'6"), 64 kilos (140 lbs)
Since Fritz was only 16 in 1728, he may in fact have put on a couple more centimeters before he stopped growing.
Fritz then put on noticeable weight starting in Küstrin, and kept it on until 1756, at which point he cut down his meals during the Seven Years' War, and remained gaunt until he died.
Evidence for Fritz's severe diet
1) October 4, 1756, D'Argens writes to Fritz that he's heard that Fritz is skipping meals, that he's "cavalierly" going 36 hours at a time without food, and that he didn't eat all the day before the recent battle. Which would be Lobositz (October 1, 1756). D'Argens is appalled and tries to convince him that eating food is important. (Remember when Suhm tried to convince him that sleep was important? I feel like this was part of being friends with Fritz. :P)
2) January 27, 1762, Fritz writes to Madame de Camas:
For four years I have given up suppers, as incompatible with the profession I am obliged to follow; and on marching days my dinner consists of a cup of chocolate.
3) January 10, 1776, Fritz writes to Voltaire:
I think one should be content with one meal per day, dinner at noon, to give the stomach time to complete its digestion before sleeping hours.
FW's and Fritz's death masks
FW's death mask:

Fritz's death mask:

Fritz's height is variously reported as 5'2" (157 cm) and 5'7" (170 cm) in the (English language) sources I've read, with little to no middle ground.
The confusion seems to arise from a difference between French units and English units. Voltaire, in his 1752 pamphlet, described Fritz as "cinq pieds deux pouces." A 1775 English translation rendered this "five feet two inches high."
Unfortunately, in the 18th century, "five feet two inches" in French was the equivalent of about "five feet six inches" in English. This is the same thing that happened to Napoleon, who was evidently the same height as Fritz, and got a reputation for being extremely short due to the failure of the English to convert, and possibly also due to his spending a lot of time around tall guards (which was probably also a factor in Fritz's case, though not as much as his father!).
In Fritz's case, the fact that he lived into his mid seventies, doubtless suffered spinal compression like most adults who live that long (it's a sheer factor of gravity), and was described as having his head bent nearly to his shoulder from arthritis in the last year of his life, i.e., the fact that he was at his shortest when he was also at his most famous, may have contributed to the descriptions of him as 5'2". But we have textual evidence that there was a failure to convert to English inches even when he was 40.
ETA: We now have the testimony of Schöning, one of Fritz's chamber hussars, that Frederick II was about 5 feet 5 inches tall. Give or take an inch, and that one is writing after Fritz died and one in 1752, he probably lost an inch or two in 34 years, from spinal compression.
Fritz's weight
Once upon a time,
Having spent several days in Saxony this week, I also visited Königstein Castle, where they have the very special big scales where August the Strong got himself and his guests of honor weighed. And thus I can report Fritz' weight in the year 1728, fully clothed, was 64 kilo.
At the time I only knew the two radically different heights, but now that we know Fritz was approximately 1.67 m (5'6"), and the lower height is due to French/English unit confusion, we have his height and weight at age 16! 64 kilos is 140 lbs, which is not underweight for that height.
FW's weight
FW, I've seen reported as 5'2" (157 cm), but that may be a misunderstanding similar to Fritz's, because I found an article in this volume (p. 38) that reports him as 1.65m.
FW's weight
According to the record at the Königstein scales, as reported by
According to the volume cited above that records FW's height as 1.65 m, his weight was 2 1/2 "Zentner" at the age of 35 (so 1723). With one Zentner being about 50 kilos, that's supposed to be 125 kilos (275 lb), according to Wikipedia (though of course units vary by time and place!).
Most likely adult heights and weights from 1728:
FW: 165 cm (5'5"), 125 kilos (275 lbs)
Fritz: 167 cm (5'6"), 64 kilos (140 lbs)
Since Fritz was only 16 in 1728, he may in fact have put on a couple more centimeters before he stopped growing.
Fritz then put on noticeable weight starting in Küstrin, and kept it on until 1756, at which point he cut down his meals during the Seven Years' War, and remained gaunt until he died.
Evidence for Fritz's severe diet
1) October 4, 1756, D'Argens writes to Fritz that he's heard that Fritz is skipping meals, that he's "cavalierly" going 36 hours at a time without food, and that he didn't eat all the day before the recent battle. Which would be Lobositz (October 1, 1756). D'Argens is appalled and tries to convince him that eating food is important. (Remember when Suhm tried to convince him that sleep was important? I feel like this was part of being friends with Fritz. :P)
2) January 27, 1762, Fritz writes to Madame de Camas:
For four years I have given up suppers, as incompatible with the profession I am obliged to follow; and on marching days my dinner consists of a cup of chocolate.
3) January 10, 1776, Fritz writes to Voltaire:
I think one should be content with one meal per day, dinner at noon, to give the stomach time to complete its digestion before sleeping hours.
FW's and Fritz's death masks
FW's death mask:
Fritz's death mask: