the fridge can be a variety of sizes and the best way to determine how big you want to go is to make a paper cutout the size of the fermenter base and place it in various fridges you want to try. on the other hand, shelves can be built into the fridge to increase space. or the side by side can be modified to increase space. here is a good thread that will answer some of your questions:
i haven't seen a fridge with a top/bottom freezer cut out to make the entire space available, but i'm sure it could be done. you'll definitely have to do some modifications to the fridge if you want to maximize the space. take the door panel off, replace it, take all the shelves out, install wood shelves, etc...
also, typing 'refrigerator fermenter' or 'fermenting with fridge', etc. in the search will yield a bunch of posts pertaining to this topic.