You can resize the image on your computer or use a free image resizer on the internet.
To resize an image on Windows, use the built-in Photos app or Microsoft Paint. In the Photos app, use the “Resize" option hidden in the side menu.
On a Mac, open the file in Preview and click Tools > Adjust Size. Set the height and width to the same dimensions to create a square.
Be careful – resizing your icon may warp it. You may need to crop the icon first and then resize it so you don’t warp the image.