I'd say find a pattern and then go from there. Perhaps find a print you just love for a focus fabric, then choose other colored prints that go with that focus fabric.
If buying just to increase your stash, I think it depends on what the fabric might be used for: borders generally need more than a yard, like several, but maybe a yard would be enough if you're making a smaller project, like a lap quilt. One Block Wonders need 6 repeats of the print. The yardage would depend on how long the repeat is. If you can't decide on a pattern, then I'd just look for fabrics you love, for whatever reason and then buy 1 1/2 to 3 yards. I generally do not buy fat quarters, but get 1/4 yard instead. Have Fun!