Thumbnail dimensions

YouTube asks for a custom thumbnail at 1280 by 720 pixels in a 16:9 frame, with a minimum width of 640, uploaded as JPG, PNG, GIF or BMP. The old 2MB file cap has been raised well past that and YouTube now generates higher-resolution versions for TV, so exporting a large clean file is a good habit rather than a waste.

Almost nobody struggles to meet the specification. The problem is designing at that size and judging the result on a 27-inch monitor, when the place it has to work is a phone feed at a fraction of that width. Export it, shrink it, and look at it there. If the subject is unreadable, no pixel count will save it.