When designing gloves, I have gathered some information on average hand sizes, though definitive measurements have been hard to find.
Here is a rough guide I have collated from various sources. The measurement given is the circumference around the widest part of the hand.
This is the information used to size my Mighty Mitts design.
Please be aware these are guides, we are all different. Where possible, my crochet patterns include guidance on how to personalise fitting.