I can put two kegs and one 5lb co2 in my 7cft. If I put the co2 on the ledge in the freezer I could put 3 kegs with room. Got my freezer at Sam's club for ~190.00.
Get this book. There is a chapter on what you are wanting. A must have for any brewer.
http://www.amazon.com/gp/product/0937381500/?tag=skimlinks_replacement-20
Designing Great Beers by Ray Daniels.
Do you also Carb your beer with gas line connected to liquid in? I just added a liquid connector to my gas line to blow back any hop sediment that gets in my uptake tube, and will also use this for carbing.