I wouldn't go bigger than a 17x9 rim with 275 tires. I put my dad's 04 Cobra rims with 275/40 tires on the back of my 68 fastback to see how they would fit, and it turned out ok. Not sure about the fronts though. 69's have a little more room than the 68's as you might know. Check the classic section or www.vintage-mustang.com PM me if you'd like pics via email.
18's. 19's. 20's. 22's. They'll all fit. The wheel diameter is not your limiting factor. The wheel width (and subsequently) /tire width is prolly what you are going to be more concerned with. Guys have been putting 28"-30" drag slicks under Mustangs of that vintage, for years. I think about 8" width (maybe 10") is about it without cutting/rolling the wheel opening. I went off on a tangent but I hope something in there helped.