That sounds weird. Most HOA's want something that looks nice and uniform with the neighborhood. Since you can't have a slab per the HOA, how do they expect you to build a decent shed that won't offend the HOA standards? No offense to those who don't have slabs, but usually HOA's want slabed out buildings. Is the no slab thing due to the ground conditions?
I think what Mulepackin did will be your only option. My shed is on a slab, and is 14'x16'. It originally had doors on it. I replaced that with a roll-up door. Custom size cost me $400 installed with a lock. I also built a shelf inside that is approximately 8' deep, and 4' below the ceiling. That took a lot of the little used stuff off the floor, Christmas decorations, kids old toys, etc. The shelf was built with scrap lumber, 4'x6's, and I used large bolts to hold it in place. Shed was covered in left over HP siding from the house. When I reside the house, I'll reside the shed again. Also, like everyone said, make it as big as you can. You will always use up the space, but once it's built, you really can't add onto it.