Well, if you want a bong that you can take big rips from, get one with a decent-sized chamber (a 3 footer or so) but since your lungs can only hold so much smoke the larger chamber/larger hits principle breaks down after a certain size. For example, if the chamber is so tall you can't light it yourself, it might be too big for you to clear in one hit and therefore won't really work any better than a smaller bong.

But of course there are many more factors to consider in your bong than size. You should also take into account things like the material, whether it has a carb or slide bowl, how much water you can fit into it, how big the bowl is, and of course how cool it looks.