Photo requirements

[ThiagoSCF]

Guys, I'm a little consufed about the photo requirements...

Frame Size:

The final frame size of the photo must be at least 35mm x 45mm.
The photographs must show a full front view of the head and tops of shoulders, with the face in the middle of the photograph.
The size of the head, from chin to crown, must be between 31mm and 36mm.
Digital dimensions are often expressed in pixels or DPI (dots per inch). The physical dimensions in pixels must be at least 420 x 540.

If I create a frame using milimeters and fit the face inside it, the final picture ends up with 100x100 pixels only. Therefore, its resolution is kinda low.

How should I approach it? Use pixels instead of mm? If so, how to make sure that the face is in the correct size?

Seniors, please advise.

Thanks