i am in the same stage as you but i find that its easier the farther back my bum is over the rear axle
also i find that lower is better than higher but i am just a beginner
a good teacher is Ryan Leech (i'll post a video when my YouTube works)
(if you liked the video, buy the DVD its all really useful)
bike wise i found manual on a Bmx easier so i think shorter the better but i am just guessing
hope you learn
Sangsta.