Your custom thumbnail image size should be as large as possible, as the image will also be used as the preview image in the embedded player. We recommend your custom thumbnails.
The Right Size of Youtube thumbnail Have a resolution of 1280x720 (with minimum width of 640 pixels).Be uploaded in image formats such as .JPG, . GIF, .BMP, or .PNG.
Click here the Link:
https://www.youtubethumbnail.org/you...humbnail-size/
