X / Twitter header size

The header is 1500 × 500, a 3:1 strip. Your profile picture sits over the lower left of it, and on mobile the header is cropped shorter still.

Two overlaps eat into a header that is already a difficult shape. The avatar covers the lower left, and mobile shows a shorter slice than desktop, so the vertical centre is the only reliably visible band. At 3:1 there is not room for much anyway — one line of text or a simple graphic is the realistic ceiling. Post images are a separate question: X shows them cropped to roughly 16:9 in the timeline and expands to the full image on tap, so a 1600 × 900 image displays without any crop at all, while a portrait image gets cut down hard in the feed.