Assuming this is for a kegerator, not a lagerator...
I went with a smallish (18 cuFt) fridge because part of the selling point for The Wife was that we would have more freezer space (little did she know that I would claim it all for beer supplies!). It's nice because I don't need an external controller, which means the freezer's cycle doesn't get all messed up.
If I were doing a lagerator, I'd probably go with a chest freezer; a little more efficient to run, IIRC, and cheaper to buy. This assumes you've got a back that can handle picking up and gently setting down full carboys. I just have a small dorm fridge right now (free), but am thinking about getting a small chest freezer to replace it when I've got a little more coin in my pocket.
Actually, if I were doing a kegerator-only setup - without needing the extra freezer space - I probably would have gotten a chest freezer. As it is, I can fit four full and one 3-gallon ball lock cornies in there; in an ideal world, I'd be able to get more like eight or nine, which I think I could do with a modified chest freezer for about the same price.
I ended up buying a cheap new fridge, too, mostly because delivery was free

. What you end up buying may depend on what's available; lots of cheap stuff shows up on Craigslist all the time.