I have one of the generic black foam ones, I really like it, mostly for my back, but legs too. I think mine is 16" or 18", I've used the 3ft ones, didn't seem to be a lot better. As far as the knobbed ones, I don't do those often, only after a more intense workout, and usually after starting with the regular one. The knob ones can be too much if you aren't used to rolling/don't do it regularly. The black high density is good, pink (or other low density) can feel nice, but I find it too soft.
Also, check these out:
Go for the foam roller first, but these can be really good.