A Pixel Specification, Not a Print One
3:4 portrait, 900-4500px wide and 1200-6000px high.
New Zealand publishes its passport photo requirement in pixels: a 3:4 portrait image, at least 900 pixels wide and 1200 high, up to 4500 x 6000, as a .jpg between 250KB and 5MB and less than six months old. There is no millimetre size to hit because there is no print.
Scans of printed photos are not accepted. The file has to be digital from the moment it is taken — the same chain-of-custody logic Germany used when it banned paper photos outright, reached without banning anything the applicant does.
There is an official online photo checker that validates the file before you start the application. Run it. It is faster than finding out after submission.