In another thread, @pdk42 stated:
I may be totally wrong, but I've been in the habit of changing the radius in USM according to the resolution of the image, my thinking being that the radius should correspond to the size of the "image unit" (whatever that means). So for images downsized to 1200 or so pixels, I use a radius of 0.3, whereas for images at about 3000 - 4000 pixels, I use a radius of 0.5 to 0.7 pixels; so if my approach is right (and that is questionable in my mind) then HR images would require even larger radii, which is in accord with @pdk42's experience. I would appreciate comments on whether I am wrong. Many thanks.I found that PP sharpening was problematical on HR photos using LR. The best I got was to increase the Radius quite a bit. Just cranking up the Amount on its own was not enough. I suspect that other raw tools might provide a better result than LR though. Try Oly's Workplace - see what that does.