The Smallest Frame Here
28 x 36mm is not a rounding of anything familiar.
Botswana's e-visa portal states that the image should be in colour and 28mm by 36mm. That is smaller than the 35x45mm used across Europe and Asia, and smaller than the 30x40mm used in Qatar, Azerbaijan and Colombia.
Because it is smaller in both directions and a slightly different ratio, a photo made for anywhere else cannot simply be re-cropped without pushing the face out of the required proportion.
That proportion is stated precisely: the face, from the forehead edge to the bottom of the chin, should be 70 to 80% of the photo. On a 36mm frame that is roughly 25 to 29mm.