I think it really depends on where the mirror is going and how much area should be visible.
I think you should be looking for about 600mm to 750mm.
Find out if it has to be wall mounted or Post fixed because the brackets vary.
I did one about 18 months ago in a warehouse. Their problem was “a junction” in the warehouse for forklift trucks. they wanted a convex mirror so the drivers could quickly and easily glance and see if anything was coming.
after a bit of digging, I ended up making my own!
below is NOT my “illustration image”. just using it to show the type of thing I did.
basically, I welded up a simple steel frame with an adjustable bracket to allow the BOW to rock side to side and up and down.
I then riveted on an 8ft x 4ft sheet of “Mirror” Dibond, which took on the bow shape very well.
once the wall bracket was thunder bolted to the corner of a wall, we tightened the rocking/moving part into place until the forklift driver could glance up in multiple directions and be able to see clearly see in the mirror, what was coming from the opposite side.
This was a very simple cost-effective solution “for us” but we charged the customer plenty for it. After all, it was completely customised to their needs, and was a BIG mirror!