Another suggestion is to have the contractor cut a control joint on the center line in each direction. One of the only guarantees about concrete is it will crack, control joints are intended to control where it will crack. Rule of thumb is 1/4 of the slab thickness or in your case cut 1" deep. If your contractor does not have the capabilities to saw cut control joints find another contractor. Preferably they would be able to green cut the slab the same day it is poured otherwise cut it first thing the next morning.
OK. I will be on a red clay caleche type soil. How deep should the substrate be and what size rock? Assuming gravel substrate. Building will not a be a garage. There will be 6' opening for a pair of 3-0 doors. There may also be one 3-0 people door. Single story with overhangs to match the house.
... I'd talk to the local concrete guys to see what others have used on that type of soil for that type of building. ...
Another little aside: In a lot of areas, "caliche" is a frequently misused term for "hard dirt". True caliche is a mixture of calcium carbonate binding other materials like rock, sand, silt, or clay. The calcium carbonate itself is whitish in color, so "red caliche clay" might just be "hard red clay", in which case my comments above about soil cement may not apply at all. Yet another good reason to simply find out what building methods have stood the test of time for the locals and run with it.
... Secondly, guys would not believe how often girls are shined on because it is assumed we don't know anything. I enjoy popping the bubble!