which size foam roller should i buy?

Answer 1

You will find some foam roller excercises described here:
http://exercise.about.com/library/blfoamrollerworkout.htm

Typically the rollers are available in 3" and 6" diameters. I think I would tend to go for the 6" for some exercises and the 3" for others.

I've never done any of these excercises, but from using various sizes of Fit Balls I know that the larger sizes are a bit more stable. Of course, instability is what you are using the roller for. So for some exercises you want the greater instability of the 3" size.