I believe posts need to be 8' on center for a commercial building. It may be different in MI. than it is in WI. so you would have to check what the building codes in your area are.
Most comm buildings use a 24" hole with a poured concrete pad for a footing, thats basically all you need for a comm. footing. You just need to find out how deep you need to go in your area I doubt it would be much over 4 feet.
An engineer in your area would be able to tell you what you need to know as far as what you need to meet the building codes.
Mueller Construction LLC.