When you paste a link into Slack, X, or iMessage and it unfurls into a rich card with a big image, that image comes from your page's Open Graph tags. Get the size wrong and the card collapses into a tiny thumbnail or, worse, shows nothing at all. This guide gives you the exact dimensions and the meta tags to use.
The short answer
Use 1200 by 630 pixels. That is an aspect ratio of roughly 1.91 to 1, and it is the size every major platform renders as a full-width preview card. If you only remember one number, remember 1200 by 630.
Platform-by-platform sizes
Platform
