LinkedIn's own recommended size for shared link and post images.
Target: 1200×627px
LinkedIn crops any image that doesn't match its expected ratio to fit its feed card, which is why an image chosen for its content can end up with its subject or text cut off the moment it's posted. 1200×627px matches LinkedIn's own recommended ratio for both native image posts and link-preview thumbnails.
This size also happens to be close enough to the general Open Graph standard (1200×630) that the same resized image will look correct when shared on most other platforms too, without needing a second version.
Both. LinkedIn uses the same aspect ratio for a directly uploaded image and for the auto-generated thumbnail on a shared link, so one correctly sized image covers both cases.
Keep any important text or logo within the centre ~80% of the frame. LinkedIn's feed card sometimes trims a small margin on different devices, and content placed right at the edge is the first thing to get cropped.
Keeping it under roughly 1-2MB is enough for fast loading in the feed — LinkedIn recompresses larger images anyway, so there is no benefit to uploading an unnecessarily large file.