Why the right size matters
Each network crops or scales an image that does not match its expected shape. A landscape photo in a portrait slot loses its edges, a small image on a large slot turns soft, and a banner with text near the edge can lose that text on phones. Starting from the expected size and ratio keeps the crop where you want it.
How to read the table
Sizes are in pixels, width × height. The ratio column matters most: an image with the right ratio can be scaled cleanly, while the wrong ratio means the network crops it. "Official" rows come from the network's own help or developer pages. "Common practice" rows come from widely used guides because the network publishes no figure. Where sources disagree, the row says so instead of picking one.
Limits such as file size and formats are listed where the source states them. Pages that publish through an API can have tighter limits than the app: Instagram's publishing API, for example, accepts portrait images up to 4:5, while the app also takes 3:4.
Resizing for several networks
One image rarely fits every network. A common approach is to keep one large original and export a square, a 4:5 portrait and a 1.91:1 landscape version from it, then add the vertical 9:16 version for stories and short video covers. PostBoost lets you attach the right version to each network in the same post and schedule them together.
This page is a reference, not a guarantee: networks change layouts by device and over time, so check the linked source when a placement matters.