Since most of my quilts are lap size, I usually buy twin size batting (about 72 x 90").
As for storing "scraps" of batting. I measure them, roll them tightly, and use masking tape to hold the roll together -- I mark the size on the masking tape. I store them in plastic storage containers (or large boxes), with a list of the sizes in the box. When I use a piece, I mark that off my list. I don't save scraps smaller than 12 x 12".