Being aware of how your thumbnail looks is just as important as what your set-up looks like up when being watched. On Twitch, the active view count is displayed on the left hand side when someone is browsing a category. If you're on the left, then you're getting covered up and your design will be obscured (you can get an idea of how that looks through these VOD thumbnails). If you're lower on the screen or smaller, that can hurt your visibility further. Personally, I like having my model be from the top of the hips up and large enough for facial expressions to be seen in the thumbnail.