Probably not the best forum for this type of question. Also, it is useful to have a link to your website and describe more precisely what you're trying to do (make the image file smaller? make the image appear smaller on the page? make the image resize when you change the size of your browser window?).
Joel Headley, Google My Business Top Contributor
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Sizing photo cover
Google's no Facebook! I struggled with this too and couldn't find a reasonable answer in Google doc's. I found it, here of all places, when searching Bing: https://havecamerawilltravel.com/photographer/google-plus-cover-photo. Maybe Google could POST PHOTO SIZES - and keep people from guessing.