it holds 4 kegs (ball lock), 10 lb co2 tank with dual body regulator, 5 lb beergas tank with dual gauge regulator. everything is packed in there though and if i were to do it over, i'd get a larger one...8-10 cu ft.
I have a 13.2 cu ft model and it can hold 8 ball locks and a 20lb CO2 tank withougt a collar. You don't need to have taps for all of the kegs that will fit in your keezer, but it sure is nice to have the conditioning/lagering/carbing space. I'm going to build a collar for mine after I move so I can put another 2 kegs on the hump just for lagering.
So IMHO, as large as you can comfortably fit into your space and budget, which sounds like 5. One of my friends has this one and it holds 5 kegs and a CO2 bottle inside.
If you go for a keezer and build a collar, then it's easy enough to keep the gas outside, leaving more space for things that actually need to be cooled.
In that case I have a huge reason to go keezer, they are liquid tight. If you have a liquid leak a keezer will contain it, but a fridge will not. Would you rather open your keezer to find it has become a beer bathtub or find 5 gallons of beer distributed across your floor? I can tell you which one SWMBO would prefer