Daihatsu Applause J Reg


I've just obtained a Daihatsu Applause (J reg - think it's 91)
It has alloy wheels. but it's missing the wheel brace. I have a 4 size wheel brace but its too thick to enter the socket that the wheel nuts are in.
I've got out my trusty socket set, the 20mm is too small and the
21mm is a little loose. Anyone have any idea as to the correct size (are they perhaps in inches or something?)
Thank you
Steve111 Wink