I store our Power Trac PT425 and all implements in an area that is 16' deep by 7' wide. I have a foot and a half on each side and a foot at the rear. I have a 60" snow blade, 60" finish mower, 48" brush hog, 5 cubic foot rock bucket, 10 cubic foot light material bucket, pallet forks and a few smaller items all in that area. If I need more space, I may put in a shelf at the end some day to stack implements.

Oh, yeah, I also store 3 six gallon fuel cans, a weed eater, many hand tools like shovels and rakes, etc... some washtubs, chains, buckets, etc... and a couple of paintings that I don't like but can't bear to part with, on the walls. Amazing what you can store in a small space.

The quick attach feature of the tractor allows me to remove ALL implements from that space in about two minutes without getting off the tractor and set them in the lawn near the shed. I can then choose which ever one I want to use.
When picking the size of your shed, you need to consider how tight you want to pack any implements and if you will be able to get them out and use them without any hassle. I would suggest waiting to build a shed until you decide on what tractor you will get and what implements you want to store inside. Then buikd a shed to suit. As MrJimi said, build it much bigger than you want.