Check out my Guide to Floor Slabs in the link below for more information but the answer to your question depends;
If the structure is detached, unheated and the soils are reasonably well draining, you don't have to insulate at all. Just let the structure 'float'
If you want to protect the slab from frost movement, you can insulate under the slab and extend the insulation out or down a distance equivalent to your frost depth.
If the structure is heated - especially with in-floor radiant, 2" of rigid insulation are typically placed under the slab and ideally at the slab edges. This insulation is primarily for heating efficiency but I'd also install the extra insulation around the outside of the structure to keep the frost from working its way under your slab and to protect the structure in the event the heat is turned off.
Hope that helps and Congrats on the new garage
