Opening, sharing, and managing OneDrive-stored documents directly within Microsoft Teams
Hello ! I'm Aymeric, and I'm here to help !
Unfortunately, on OneDrive web, you cannot enlarge the thumbnails of your pictures. Once you selected Thumbnails in Display options, you cannot do anything more.
However, here's a workaround that might work out for you : if you're able to sync OneDrive content to your Mac, you will be able to see your files in your Mac's Finder, and then resize thumbnails using the tools present on your Mac.
Feel free to ask if you have any other question !