73 Duster Mini Tubbed - How big a wheel?

17x12, 335/35/17 will fit. That's if the rails are the first interference point or at least even with the springs. Measuring for the rear BS is easy and I wouldn't take anybodies numbers without measuring first

In a 17" that's about as big of a tire they make. I have a 345 in the rear of my demon which fits with a slightly trimmed quarter lip (19x12)