If you're looking for some sort of consistency in terms of the size of the folded fabrics, you might try going for 9" (a quarter-yard) as your "base measurement".
Take that quarter yard, and fold it in thirds.
Have a half-yard? Fold it in half, now you're down to a 9" piece; fold that in thirds.
For a full yard, fold it in half, and in half again; now fold that in thirds.
Basically this is what we do when we're packing up fabrics to ship in the flat-rate envelopes. We want them all to turn out to be pretty much the same size when folded too... just our reasons are different than yours! :-)