Question about photo specifications

[yodnokzo]

The photo specifications on my application are:

• The photo must show a full front view of the head and tops of shoulders, with the face in the middle of the photo.
• Size of the head, from chin to crown, must be between 31mm and 36mm.
• Image size: at least 420 x 540 pixels.
• We accept JPEG or JPEG2000 format.
• File size: approximately 240 kB, no more than 4 MB.
• Image must be in colour (24 bits per pixel) in RGB colour space, the common output for most digital cameras.

The height of the image should be at least 540 while the height of the face must be between 31mm and 36mm. I converted 36mm to pixels using online calculators. According to these calculators, 36mm approximately equals 136 pixels. However, it doesn't make any sense for the height of the face to be 1/4 of the height of the photo. The calculator assumes that PPI is 96, but there is nothing about PPI in the conditions. Does anyone know what should the height of the face be in pixels? Thanks!