I have everything from whole bolts of fabric to small fat quarters.
It just depends on the fabric, if it is something I will/can use in lots of quilts as it is a neutral that will not go out of style and its a great price ... I buy lots , sometimes the bolt. Becasue when in doubt they can be backings.
If its a trendy fabric but exceptional , mimium one yard maybe three depending on price. This depends also on wher ein the quilt I think it may end up . if it is going to be a border fabric , Three yards.
I do tend to look at the fabric and how difficult it will be to find compatable fabrics. This was a lesson learned the hard way. Some fabics are spectacular but are very hard to get enough compatable fabrics to make a interesting quilt.