A "safe" fit would be 225/70/15 or maybe 235/70/15. "Best" fit requires some measuring for backspace and tire width. I figured it out by borrowing a test wheel and tire in desired diameter and measuring front and rear... and purchasing tires/wheels based on measurements. If you are going for same size front and rear, you may be able to fit bigger tires if that is what you want but backspace needs dialed in.

Suggest you "eat your pride" and drive it home with whatever tires you have and then borrow a 15" test wheel/tire for measurement... and order accordingly.