PMB says:

I would say 5 mm. My fiance tried on lots of the same style (milgrain edged) in 4,5,6 and 7 mm styles and liked the 4 mm the best, but he has lean fingers for a guy. 5mm seems average to me.
I think it depends on the size of the man’s fingers, for what width looks best. My fiance is only a size 7.5 so a 7mm ring would look too wide, but the 4 mm looks perfectly proportional. A guy with wider fingers, like a size 10 or 11 might look better in something like a 6 or 7mm ring.
Just go try on a bunch of different width in the same style to compare when you like best.