If you are on tripods, a 4" slab with #3 rebar at 16" both directions is fine. Remove the top 6" of existing soil to get rid of loose and organic matter. Backfill with sand, crushed limestone, or whatever is typically used locally in your area. If you go more than 120 square feet, I'd put a grade beam around it. Grade beam would be something like 10" wide and 18" deep or more with 2 #5 top and bottom. I should do some standard details for anyone to use. Won't be able to get to it this week but I'll put it on my list.
If you are doing concrete piers, isolate the piers from the slab (don't let them touch).
I was actually thinking about doing several steel piers to the slab so I assume it would be better to do mini slabs for each one with motel o scopes on top? Separately what about in a large roll off roof? One main slab and then isolated ones for each pier?
Edited by akulapanam, 16 April 2019 - 10:19 PM.