The Twitter header, still used on X, is 1500 × 500 pixels. The profile photo covers the lower-left. The circle marked Avatar is that overlap, so keep titles and logos out of it and toward the center of the banner. Some phone layouts also crop a little off the top and bottom. Drag and zoom, then download JPG or PNG. Processing stays in your browser.
Sizes for YouTube, Discord, Twitch, and Twitter are in one guide. Read: Common social media banner sizes and how to make them