This subject has come up before. The example of Fritz Hoffmann shows that the German air service probably did NOT have a "minimum" requirement for pilot height; at that point, late in the war, even an elite unit like
Jasta Boelcke accepted him as manpower was becoming limited indeed.
Yes, the book about Josef Veltjens by his son Klaus states that, when Seppl became a pilot in Flieger Abteilung 23, "He selected the shortest man, Ltn. Walter Gnamm, as his observer (or "Franz") to balance his own tall man's weight." Such things were a real concern in those days.
Of course, many of the most successful airmen were of small stature. Udet,
Hans Klein and Anthony Beauchamp-Proctor come to mind immediately. In Neal O'Connor's wonderful final book, he wrote that Jasta 6 ace
Franz Hemer rarely spoke of his wartime exploits to his family, except "..To say that his small stature probably saved his life after he had become a fighter pilot. Because he weighed less than most young men of his generation, he got a bit more speed out of his aircraft (he was referring to the Fokker Triplane at the time) and he also could fly a bit higher than the others and could attack from above."
There were a number of men with prosthetic devices or other handicaps that flew on both sides in WWI - "Timbertoes" Carlin for the RAF and Walther Karjus in Jasta 11 and 75. And of course, there's Berthold and Nungesser, who persevered and did very well despite crippling injuries...
Men of above average height (like Veltjens) could also do quite well, as noted. IIRC, Hugh "Dingbat" Saunders of No. 84 Sqdn was one such, and I believe he told interviewer Jon Guttman that his large size and weight were something of a detriment to the performance of his SE5a.
Finally, just for fun, many German regiments liked to find their tallest and shortest members and have them pose together for a "Long and the short of it" sort of humorous photo. Here's one from a German two-seater unit. Neither of these gents were pilots of course, but it does illustrate the extremes that were accepted in wartime units.