Whatever you want to rubber band, whatever model plane, take it right into the office supply store and back to the rubber band rack.
The STAPLES near the house has one bag of each size rubber band open for inspection.
Test 'em until you find the perfect fit.
The bag will probably be $1 (that's one dollar) and some change.
And the turnover in an office supply store almost guarantees you that the rubber bands you're buying are probably only days old.