Height
I'm 6' 4". I haven't met many aircraft that I can't get in to, though some are a bit more of a compromise than others.
One thing I have noticed is that "normal" size pilots/students are more prone to complaining about ergonomics / cockpit fit. Taller pilots are just grateful to be able to get in and are more likely to accept the compromises that may be required.
A restriction based just on height is over-simplistic. The relative proportions of our bodies vary significantly so a "no fit" for one person may be fine for another of an equal height. When there is a question over whether someone will fit in the cockpit I ask them to come to the airfield just to see if we can get them in, if they fit then we book them a flight.