A Pixel Floor, Not a Print Size
400 x 600 minimum, face at about 70-80%.
Iran's e-visa portal gives the requirement in pixels: at least 400 wide by 600 tall, with about 70 to 80 per cent of the photo filled by the applicant's face. No millimetre size appears, because the submission is a file rather than a print.
Both figures are minimums, so a larger image is fine as long as it stays under the file ceiling. What matters is the proportion the face occupies.
The background must be plain white or off-white, and the photo must be in colour and in JPEG format — .jpg or .jpeg.