How the photo gets under 5 KB
A file this small has room for far fewer pixels than a phone photo has. The tool first lowers the JPG quality; when even its lowest quality is over 5 KB, it makes the picture smaller step by step until it fits, and says so under the preview. Check that a signature's strokes or a face are still clear before you upload.
The download is a new file made in your browser from your photo. Your photo is never uploaded, and location and camera details (EXIF) are left out of the download.
KB or KiB
Here 5 KB means 5,000 bytes. Some forms count a kilobyte as 1,024 bytes; for them choose KiB in the unit menu, and the limit becomes 5,120 bytes, 120 bytes more. If the form does not say, keep KB: a file under 5,000 bytes passes either way.
If the form also sets a minimum or a pixel size
Open Form requirements and format and enter the smallest size the form accepts, its width and height in pixels, or both. A photo that is already smaller than the minimum needs the opposite job: increase image size in KB saves it again with less compression. Meeting the size does not mean a form will accept the photo, so check its other rules too.