They list a 5 5/8 head in the classic parts catalog. They also sell arms.
Ron,
Classic Parts round mirror is listed as 5" diameter. Not sure where you found the 5-5/8" dimension???
The "black painted" mirror in Mike B's reply (and quoted by 68ironhead) shows the shipping dimensions as 6"x6"x3". Not sure if that equates to a 6" diameter mirror itself? As Mike B mentioned, a call should clear up the details.
They do offer this
Stainless Steel round mirror that specifies it is 5-5/8" diameter.
Blue 50,
Will be interested to hear what you selected and how it works out for your.
Dan