Well, it's the biggest that will fit under flex. I've got 33"s on mine right now, but that's cuz the tires that were on it when I bought it were dry-rotted to hell, and all I had was this set of 33"s. (You Might Have A Problem If ... you keep spare sets of tires around

)
Anyway, mine won't be seeing trails -- I've the '74 for that -- so the 33"s won't likely rub whilst mall-cruising
-- A