I'd take a look on Craigslist too. I found a 5 cu ft chest freezer that works perfectly for $40. It will fit one 6 1/2 gal carboy and one 6 1/2 gal bucket both with air locks. But if you have the budget, I'd get the biggest size you can fit/afford. You'll want the extra space.
Edit: After re-reading your post I realize this might not be the advice you were looking for, but I'm not sure you could fit it in anything smaller than 5 cu ft.