My garbage grade B liitokala's came with 6mm, 30% were stripped out so I tapped the trashed holes to 8mm.
Ive got Docan Power Eve105Ah. They have 4mm threads!
My new Eve LF280K cells have a 6mm welded studs. So id say 6mm is pretty common.
Now I am not an EE, but maybe someone here is...Good...