I'm not aware of any specific standard, but of the several speakers that I checked, including Axioms and a Paradigm, the diameter to the outer edge of the metal frame was included.
Odd that you asked, because I'd always assumed it included the diameter to the outer edge of the rubber surround, but apparently not. It's to the outer rim of the speaker's frame.
Just be happy you don't have to deal with centimeters and millimeters (plus inches!), which I had to contend with when I edited equipment directories in Canada.
Axiom Resident Expert