Many application portals, college admissions forms and ID-upload fields ask for a photo or document scan under 50 KB.
A 50 KB JPG holds a clear head-and-shoulders photo at about 600 pixels wide. For pages of text, crop the margins first so more of the budget goes to the content.
Forms use many different caps. The related pages cover the neighbouring sizes, and the FileHost.net Image Compressor gives manual control of quality and width.