I originally bought 1 yard of a fabric I liked. Then I cut down to 1/2 yard. But then I found that I was missing lighter coordinating colors, so I usually get 2 yards of those.
If it's something that I know is for a single quilt, I try to get enough for that quilt. I rarely make anything smaller than queen size, except for baby quilts. Even the quilt for my parents' double bed is queen to king size. When they are no longer with us, it will fit the larger beds that most people have. I just wanted it to be small enough that it didn't drag on the floor.