100% Private100% PrivateImage Guide
WordPress will accept any image size, but it will not make a slow image fast. Uploading a 4000-pixel photo as a featured image, then letting the theme shrink it, still costs the full download. The fix is to upload at the right size in the first place.
Use these presets as a starting point: Featured 1200 pixels, Product 1000 pixels, Header 1600 pixels, Thumbnail 800 pixels, all in WebP at 80% quality where your theme supports it.
A featured image at 1200 pixels is wide enough for most themes and far smaller than a camera original. A thumbnail at 800 pixels is still sharp on retina screens, because thumbnails are displayed smaller than that.
This tool is a preset workflow, not a WordPress plugin. It prepares the files in your browser, and you upload the finished files to WordPress yourself, which keeps your site free of another plugin to maintain.