I had an 8x12 garden shed that I used for bike storage. Built it myself and used landscape ties as "sleepers" to put it on. I presently have an 8x8 shed and it's too small, fortunately I also have a garage. Here are some things I would want to ensure if I were you: 1. Electricity. Can you run power to it? If so, that will make a big difference otherwise you would need windows for light and windows will create a security weakness. 2. Security. You will need to make sure you have a good strong door with a good lock on it. The sheds you buy often have weak hinges and locks that could easily be broken off. 3. Zoning. There are probably rules that dictate how large a shed you can have and if there are setback requirements. Look around your neighborhood to see what other people have. Ask.
Good luck.