The magic numbers are 22 by 14 by 9 inches: that's the max size most airlines allow on board. From there it's personal preference: if you're heading someplace where the paved roads end, consider a bag with hideaway backpack straps. If you're an organizational freak, get one with lots of pockets.
WHY THIS ONE: Unless you plan on day-tripping with it, choose a hard-sided case like the 2,275-cubic-inch Tumi Vapor International. It's reinforced with the stuff used in bulletproof glass, safeguarding sunglasses and cameras and easily transforming into a makeshift stool at the gate. Smart: TSA-certified locks mean screeners can get it open without damaging anything. Yes, it's pricey, but it will last decades.