I buy the largest size I can afford (an entire roll is out of the question right now) and keep anything leftover. Even if I am only going to make a lap quilt, I buy the king size batting coz I can usually get 4 lap size from 1 king. It is definitely more cost efficient. Small pieces can be put together with zigzag stitch or iron-on tape - once in the middle of sandwich you can't tell it was pieced.