Use what's confortable for you.
Terina:
I agree with you about ME being a great workout. I own a regular 5"-8" step, and also this thingly adjustable step I bought ages ago from the Firm. The latter step adjusts from a 10" to a 14" step. For Cathe's leg presses on ME, I use the Firm's 10" step, since I find the 14" step too tough on my knees, and too hard on me in general.
My advice is to start with what seems confortable with you -- then work up slowly. This is also true with the poundage.
-- David