A useful LinkedIn portrait shows a face clearly without requiring the viewer to open the picture. Start with a well focused photo that has enough light and a background that does not compete with the person. If the face occupies a tiny part of a group photograph, cropping it tightly may reveal that there were never enough source pixels for a clean headshot.
This page selects our 800×800 square preset. It is an export choice for preparing the image, not a claim that LinkedIn displays your avatar at that size. The visible portrait is often much smaller, so judge expression, framing, and recognisability at normal profile size.
Leave room around the head and check how the square would look with rounded edges. Avoid a crop that removes the top of the hair just to make the face larger. A comfortable head and shoulders composition usually reads more naturally than an extreme close-up that feels cramped beside the surrounding profile information.