YouTube thumbnail size and dimensions guide
YouTube recommends custom thumbnails of 1280×720 pixels with a 16:9 aspect ratio and a minimum width of 640 pixels, in JPG, PNG or GIF format. The long-standing file size cap is 2 MB, but limits have been changing, so check YouTube’s upload screen for your current limit. Designing at 1280×720 avoids blurry scaling.
Keep important text away from the bottom-right corner, where the video duration label appears. Check YouTube’s Help Center for the latest requirements, since limits can change.