This depends on the size of the page that is going to display the FAQ - we suggest a maximum width of 900px.
You can adjust this either when you create the image (i.e. take a screenshot) or by editing the size of the image in the Image Properties box on upload.
Important: By making sure the lock icon is set to closed as in the above image, you will keep the correct aspect ratio, making sure your image does not look squashed or stretched.