The Twitter / X Header size (1500×500px)
The Twitter, now X, profile header spans the top of your page at 1500x500 pixels, a wide 3:1 banner. Uploading at this exact size prevents the platform from stretching your art, and the generous width gives you room to reinforce your brand right where visitors land before reading a single post.
The overlap to plan for is the circular profile picture, which sits on the lower-left of the header and covers whatever is behind it. On top of that, X darkens the header slightly and how much vertical area is visible shifts between desktop and mobile, so the top and bottom edges can be shaved depending on the device.
Design a safe central zone: keep taglines, handles, and logos away from the bottom-left where the avatar lands and away from the extreme top and bottom that mobile may trim. Because the header sits behind your name and bio text on some layouts, a clean image with breathing room reads far better than a busy one crowded with small details.