For a first boa, there isn't much that can beat a normal columbian, in my opinion. They don't get as large as many red tails, they're very docile and easy to handle, and they are very forgiving of beginner mistakes on things like humidity and feeding (not that you can freeze them or anything, just that they're hardy and unlikely to keel over on you if everything isn't 100% while you're learning). As for the 6 foot issue, males are overall smaller than females (although it varies from indevidual to indevidual), and really even if it goes a little over 6', the difference between a 5'6" snake and a 6'6" snake really isn't all that noticable. It's very unlikely that a columbian is going to get unmanagably large, especially if you feed it reasonably sized meals on a reasonable schedule while it's growing.
To answer your other question, they can really eat any rodent, but I would stick to rats. Rats are very nutritious, and the snake doesn't need variety in it's diet for either nutritional or gourmet reasons. If you just don't like rats, mice work fine, I hear gerbils are O.K., and many people feed their large boas guinea pigs or rabits. Really, though, it's very easy and healthy for the animal if you just stick with rats.