A LinkedIn post can run to 3,000 characters, but the feed shows about three lines before folding it. Type or paste your post: the page wraps it at the feed's real widths and font size, and shows exactly where each fold falls.
The background image is a 4:1 strip. On desktop the round profile photo covers its lower-left corner; the app crops the strip's sides and puts the photo lower. Open your banner to see both, with the hidden parts marked, and save it at LinkedIn's size.
LinkedIn turns an uploaded document into swipeable pages. Every page is shown at the shape of the first, so a square or 4:5 PDF fills more of a phone screen than a 16:9 slide deck. Write one slide per paragraph and the page builds a real PDF at the shape you pick.