Discord uses two banner sizes. A profile banner is at least 680 × 240 pixels. A server banner should be at least 960 × 540 pixels, at 16:9. Pick one, then drag and zoom the photo. On a profile banner, the circle marked Avatar is the lower-left area your profile picture usually covers, so keep titles and logos out of it. 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