I keep a note on my phone with these numbers because I could never reliably remember them all, and I'd guess most people running more than one account do the same thing. Here's the current version, plus a bit of the reasoning behind the ones that trip people up.
Shared photo: 1200 × 630px, works well for both feed posts and link previews. Cover photo: 820 × 312px on desktop, though it crops differently on mobile so keep anything important centered. Profile photo: 170 × 170px, shown as a circle. Facebook is fairly forgiving on file size, but anything over 1–2 MB will still slow the upload down on a weak connection, so there's no good reason to skip compressing it anyway.
X, formerly Twitter
In-feed image: 1600 × 900px, that's 16:9, and it displays cleanly without cropping. Header photo: 1500 × 500px. Profile photo: 400 × 400px. X compresses fairly aggressively on its own end, so starting from a clean, well-compressed JPEG rather than a huge PNG generally holds up better through whatever processing it does.
Shared image: 1200 × 627px for link-style posts, or 1200 × 1200px square for a standalone photo. Cover photo: 1584 × 396px. Profile photo: 400 × 400px minimum. LinkedIn's feed tends to favor taller images with more vertical presence, similar to Instagram — a square or slightly portrait crop usually gets more visible space than a wide landscape one.
The platform is going to crop your image whether you plan for it or not. Planning for it just means you get to pick what stays in frame.
Standard Pin: 1000 × 1500px, a 2:3 ratio, and it's what Pinterest actively recommends and favors in its own algorithm. Square Pin: 1000 × 1000px, fine for product shots. Pinterest is built around tall, scrollable images more than any other platform on this list — a wide landscape photo gets visually squeezed and tends to just underperform compared to the same shot cropped to 2:3.
Across all four of these, exporting at the platform's own target dimensions beats uploading your original and letting its algorithm decide the crop and compression for you. You get a more predictable result, and in most cases a sharper final image, because you controlled the compression instead of leaving it to the platform.
You can hit any of these exact dimensions with ImgReduce's "by dimensions" mode — type in the width and height and it handles the rest.