I was curious about the difference in energy usage between a mini-fridge and a full size fridge and did a quick look at some sizes. The smallest was the best but not by a very significant amount. I also looked at one of the largest (and expensive) that was "energy star" and it was significantly lower than the other large sizes. I did not compair brands, just sizes, so this is a very non-scientific list.
1.7 Cubit ft $34/yr 318 kWh yearly usage
4.6 cu ft $37/yr 342 kWh
14.8 cu ft $47/yr 443kWh
18.2 cu ft $51/yr 479 kWh
23.7 cu ft (Energy star) $41/yr 388kWh
So I'll let the wife decide if I get a used full size or modify a mini.