Twitch panel size
Height is flexible: 300 is the convention, not a limit.
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Panels are 320 pixels wide. The height is up to you. 300 is the convention, but a schedule panel at 320 × 600 is perfectly valid and often more useful.
How to use the twitch panel size
The fixed width is the only hard constraint; Twitch scales panels to 320 wide and lets the height follow. That makes the common mistake obvious once you know it: a panel designed at some other width gets scaled and any text in it softens. Design at exactly 320 wide, or at a multiple of it if you want a sharper result on a high-density screen. Panels stack in a column beneath the player and each can carry a link, which is what most people use them for. Socials, donation links, a schedule and a gear list.
Questions
320 pixels wide. Height is flexible; 300 is the usual convention.
Yes. A schedule panel at 320 × 600 is common and works fine.
It was probably designed at a different width and scaled to 320. Design at exactly 320, or an exact multiple.
About 2.9 MB per panel.
Yes, GIF panels work, though they are visually noisy in a stacked column.