You want the pedal to be a couple inches short of your fullest leg extension at the bottom of its cycle. You shouldn't stretch your legs as much to hit bottom on a unicycle as you do on a bicycle, because you need more control on the unicycle and your leg muscles are weaker at full extension than mid-range. If you can put both feet on the ground while sitting on the seat with the unicycle vertical, the seat's much too low and your legs are cramped when at minimum extension. Raise the seat a few inches and try again. Incidentally, almost any unicycle can have the seat raised to accommodate your height.
-from Juggling Information Service FAQ