I use 5/8 x 3/16 as I like the size and grip, but I've seen others use all different sizes. As to how many again depends how quickly you want to build, 20 would get you started, I have over 150.
Lots of videos on youtube for generic stormtrooper building, remember your building a ROTJ which will have some differences like overlap method, not but join and cover strip, links to Mark's (sskunky) video already posted by Alan in post above.
I advise looking through some of the other ROTJ build threads, you'll be able to find lost of helpful build info there.
Always check with the CRL's (costume reference library) these do undergo updates from time to time and there are some pending additions at the moment, you can find ROTJ Stormtrooper here