No legal requirements I know of. However, a couple of observations. There is a significant variation in diameters of tyres with the same nominal size. Here are two 195/50R15 tyres:
Also, 195/50R15 and 195/55R15 are both acceptable with the Sierra speedometer and drivetrain, the speed difference is only about 3%, but the diameter is different by about a centimetre.
In other words, I would leave an inch or so if I were making new wing stays.