Looking at your post it appears you don't currently have a tractor? If you are looking to buy, the bigger they are the wider the stance.
You need to define how much weight and how much HP you need for the task you perform,and then look at the width. Look for a tractor that has two piece rims that can be reversed to change the width. You do realize that on a given tractor the width can only be modified by six inches or so, not much help in the thick stuff. The turning radius will be what it is, again, the bigger the tractor the greater the radius.