Pontius Navigator is quite right and it might even be me he is talking about. My sitting height was too big for the Gnat, so I was not accepted for pilot training, even though I was really only interested in flying heavies.
When I tried to re-role, the safety equippers had to fit a new dinghy attachment strap on my LSJ in order to lengthen it by about 3 inches.
Your body proportions have to be a fairly normal.