Twenty inches is pretty good for learning. You wouldn't want to have anything bigger than a 24 inch or it would be more difficult. I would put your seat so your leg is almost straight when your pedal is at the bottom.
I saw where you said, "I bought my first unicyle today!" That spurred me on to respond quickly. Now I see that you have been unicycling since 4/13/09! That means your a pro
in comparison to me. I got out on in a parking lot for about 45 minutes last Friday and
I am using two sticks like ski poles to balance as I move about.
It was really tiring to do it for so long. I Kept going for maybe forty or fifty yards at a time but it was move a bit, stop and balance, move a bit more. I never really got going steadily or felt I could drop the sticks.
I am on my way out once more to see if things are better today. I need to be riding without my sticks by Thanksgiving.
Almost forgot, I have a twenty inch wheel. Is it easier to have a bigger wheel?
And, how high should I be setting my seat. Should I be able to touch the ground when I am sitting straight up or should my toes just be on the pedals?