YouTube recommends channel art at 2560 × 1440 pixels. Upload a photo, then drag and zoom the crop. The dashed box labeled Text safe is the part that stays visible on phones, tablets, computers, and TVs, so keep titles and logos inside it. Download a 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