I would say that width may be more important than radius, to some
extent. You need the guard wide enough to extend at least a bit beyond
the chuck jaws. Secondly, the guard should prevent coolant from being
thrown at you, so it needs to either go lower than the chuck's
middle, or else you ned to add a rubber splash guard to it. My guard
is for lathes with 15" swing, and my lathe is 13". It seems to fit
reasonably well.

i