Forms often say things like "photo 200 x 230 pixels" or "below 50 KB". Two steps solve most cases: first set the pixel size, then, if needed, reduce the file size.
Steps
Read the form rules and note the pixel size and the largest file size.
Open the Resize Image tool and add your photo.
Type the width and height the form asks for.
Download the resized photo.
If the file is still too big, open Image to Exact Size (KB) and type the limit.