My Ettco accepts up to 8 mm shanks so I measured the squares on all my 5, 6 and 8 mm taps and between them there were 7 sizes, 3 metric and 4 imperial even though they are metric taps. here is a list in order of size;
5/32", 4 mm, 11/64, 3/16", 5 mm, 6 mm, 1/4"

I have formed an idea for a simple single piece design for a square driver, which should solve the slipping problem that I get sometimes and only take 1/2 hour to make.

My other tapping head, an AH made in the US, takes up to 1/2" shanks which means up to 12 mm for me.