I have a crusher run gravel floor in my shop......Don't do it. equipment will not have a sturdy base, and its hard on the feet. Moisture comes right through the gravel and can make it rain inside if its closed in. I will say this, figure out the size you need then double it. I am switching to a 16x24 shed with double floor joists and 16 inch centers with 2 lofts. I'm in Oklahoma so insulation is a must have. Last year it got to 114 degrees.
Lee