A 7.2 cu ft chest freezer will fit 4 cornies and probably the co2 tank on the hump ( I have a 5lb tank so I don't know about the 10 lb one ).
If you want room for a carboy as well, I'd say go with a 14 cu ft chest freezer. Should be plenty of room. I know there are some 9 cu ft ones too. Homebrewtalk has a thread on their forum regarding sizing your chest freezer. They suggest making cardboard circles of the kegs ( and carboy in your case ), taking them to the store, and placing them in the chest freezer to see how they will fit.
My chest freezer is a GE, and it is running great so far.