Application forms are strict about photographs: an exact width and height, sometimes an exact file size, and they reject anything that misses. Here is how to hit the numbers exactly.
Forms state a size in millimetres or pixels, so that is what people set, and it is the requirement least likely to fail. What actually gets photographs refused is scale: authorities also specify how much of the frame the face must fill, usually somewhere between 60 and 80 per cent from chin to crown, and a photo that is exactly 35 by 45 millimetres but framed too wide is rejected anyway.
That is what the two guide lines are for. Line the crown up with the upper one and the chin with the lower one and the proportion comes out inside the band your document asks for, with the millimetres handled at the same time.
One limit worth knowing: enlarging a small photo cannot invent detail that was never captured. If the form wants 600 by 600 and your original is 200 pixels wide, the result will look soft no matter which tool you use.
Open the saved file and look at it at its real size. A 35 by 45 mm photo saved at 300 dots per inch is 413 by 531 pixels, which is the figure many online forms ask for. If the form quotes pixels rather than millimetres, that number is the one to match.
Some forms also set a limit on the file size, often 200 KB or less. A photo at that size is usually already small, but if yours is over, run it through Compress to a size afterwards and choose JPG. Do the sizing here first and the compressing second, so the compressing never has to guess at the shape.
35 by 45 millimetres is the most common print size, and forms often ask for 600 by 600 pixels or similar. Use the number on the form you are filling in, not a general rule.
No. You are keeping fewer of the original pixels, not rebuilding them. It is enlarging that costs quality.
No. Choosing the document sets the millimetres and the print resolution together, so the pixel size follows from those.
No. Every step runs on your own device.
Make the photo here first, then use Compress to a size and type the limit in kilobytes. Keep JPG as the format, because many official forms reject WebP.