Common sense suggests smaller ones would be somewhat overdriven and largers underpowered but I'm not sure. My first ballast I found was a Keystone one out of a fire-damaged Lithonia shoplight that was suitable for everything from F15T8 to F40T12! I thought it would make a good universal tester ballast but then it turned out to be nonfunctional and I found my Sears shoplight...with a full power GE Bonusline HPF PCB containing ballast, the good old stuff! n
The Fulham ballasts are really cool because they have like a dozen wires and you connect a certain number for certain lamps so it actually powers its range of lamps all quite well since shorter lamps call for fewer wires and longer lamps call for more wires so they're all powered pretty close to full power.