LinkedIn link and share images: 1200x627, the 1.91:1 shape their preview cards expect. Drop an image; it comes back at that exact size, cropped where you choose.
Where this size applies: the preview card when you share a link, and standard landscape image posts. It is the size LinkedIn's documentation recommends, and the one that stops a shared article turning up with its thumbnail cropped through the headline.
Where it doesn't: LinkedIn's feed also takes portrait images up to 4:5, which occupy more screen on phones. This page is for the landscape card; if you're posting a photo rather than a link preview, portrait may serve the feed better.
Text in the image? Keep it away from the edges. The card renders at different widths across desktop, mobile and the app, and edge-hugging text is the first casualty of a resize.
The sizing here is exact. LinkedIn scales what you give it; giving it the native size means the sharpest possible card, with nothing left to their scaler's judgement.
Batches work, and each file reports its final bytes on the way to 1200x627.