Most newer chest freezers have a "shelf" where the compressor is. This shelf can hold 5lb CO2 bottle.
A Pony Keg is about 17" in Diameter, and a corny is 9" in diameter (see
http://www.micromatic.com/beer-questions/sizes-keg-draft-beer-available-aid-11.html). That won't fit in a 7cuft chest freezer, unless you build on to the shelf and store the CO2 tank on it's side underneath it, but I don't recommend it. Check out Thirsty's Keggerator thread. He built a nice collar that will allow you more room in the chest freezer as well as an easy way to route the CO2 lines into it and keep the tank outside, not to mention front mount taps which are MUCH more convenient, and cleaner than top mount (ask me how I know... I've been DYING to say that!). I'd recommend you go to the store and take a tape measure with you to find the best model for you though.
Rob