Width or height, not both, keeps the shape
A passport-style box wants both numbers and Fill or Fit. A banner that must stay wide wants one side and the ratio left alone.
The server cap
The long edge on the server is 4096 pixels. A camera file larger than that is scaled in the browser before upload.
Pixels are not kilobytes
A resized photo can still be too heavy for email. Save it, then open Compress to KB if the host rejects the file.
Format after the resize
JPG for photos, PNG when you need a transparent edge, WebP when the page can take it.