You've uploaded your photo, filled every field, and then the form stops you at one small box: "Signature size should be between 10 KB and 20 KB." The picture you just took of your signature is over 1 MB. Or worse, you shrink it and the signature turns into a grey smudge nobody can read.

A signature is one of the easiest images to get small, because it's mostly empty paper and a few lines of ink. The trick is to throw away the empty paper before you compress, not after. Here's how to do it on a phone or a laptop, plus what happened when we tested it.

First, find out what your form actually wants

There is no single "correct" signature size. Every job portal, university and testing body sets its own rules, and they change from year to year.…