Hi, the dimensions of the cover art for itch.io is 630x500px, for SoundCloud it's a square (like normal cover arts) and if you want to fill the whole screen for YouTube it's typically 1920x1080px. It is ultimately up to you, but if I were to recommend a size, I think a square or 1920x1080px would be easiest to use and resize for most sites. Hope this helps!