I'd build it about 16X20, but only part of it...say 16X12 would have a floor in it. The other 8 feet I'd leave with a bare dirt floor or put sawdust in it. I'd have an internal door between the two rooms of the building and an external door on the larger room. The 8 foot side with the dirt or sawdust floor is where I'd do my skinning and fleshing. I know from personal experience that it sucks getting blood on a wood or concrete floor. If you're careful you could also boil, dye and wax your traps in there with a small propane stove. The larger room is where I'd put my furs to dry and store most of my material, supplies, etc. My shed would have electrical, good lighting and insulated walls. I'd also have a barrel stove in it to keep me warm. A window or two wouldn't hurt in case you get something stinky in it. You might be tempted to build it smaller, but I guarantee you after you start using it you'll wonder why you didn't make it larger.