What are the 9V battery terminal size specifications?
They're specified in the relevant standards, such as IEC 60086-1, which are not free. Fortunately, our friends at the Kenyan Bureau of Standards published a public review copy of the standard:

The relevant size for the 9V battery is the &quot;Minature&quot;.
The male post is of specified dimensions, the female merely has to fit the male (OC render).

And, for completeness, here are the overall battery dimensions

If you are going to be signing off on tooling to make 1,000,000 snaps, it might be wise to double check against the actual latest standards, but as this battery design is &gt;60 years old probably not much has changed other than conversion to metric.