Thanks for the share on the dimensions of the uploaded image. I'm not positive if Kens Template set resizes the GR3 image or not but I've changed mine so we'll know soon enough It is probably fine as is but to my eyes it could stand to be a touch larger, but that's just my aging eyes.
I use the narrow Saratoga template as well and have set up the GR3 page to show my GR3 images at the width of 620px, rather than the default width of 512px. So, in addition to changing the default publishing settings on GR3's main screen on your home computer, make sure the GR3-radar-inc.php script for your website is also set to 620px on lines 23 and 24. That hopefully should do the trick.