Just did this
I just replaced the standing rigging on my '88 33.5 last year. There were 3 different sizes, the largest was for 1/2" studs. I would reccomend inspecting your own boat and not going by mine because they could be different. For instance, the new backstay turnbuckles they used were different from the old ones. If you are replacing the turnbuckles, shouldn't you also replace the wire rigging and studs? Or is the wire newer than 15 years?