rtnx:
I have no experience with the P90x2 kit, but most cylindrical full-size round foam rollers are 6"x36". However, you can also find them half-round and/or in 12" or 18" variants.
I personally own two, one 6"x36" high-density roller and one 6"x13" TriggerPoint 'Grid' roller.
Do you use the triggerpoint roller with p90x2?