Think about the base/floor being at least 4” above the dirt.
Think about the base/floor sloping down towards the door.
If you don't want anything living underneath your shed, put it high enough off the ground for the little critters' predators too. You'll end up having neither. Your excavated base should be a foot wider than each side. Where you live will determine how deep your excavation needs to be. Top it off with a few inches of gravel and nothing will grow or live underneath, it will not get muddy, plus you will have some useful, dry storage underneath. In most municipalities, sheds over 120 square feet need to be permitted and may have set back requirement from your property line.