For an AWD vehicle, all of the tires must be the same overall diameter so that the differentials in the car are not overworked.
In case you were wondering, my parents just bought an Impreza and we were talking about tire sizes for it just before winter started, so we went outside and snagged the tire size off the spare. It's overall diameter was nearly the same as the original tires on the car itself. Your Legacy should be no different.
If you're still unsure, it's a simple matter of taking the size of the original tires and the size of the spare and calculating their overall diameters.