The ideal YouTube thumbnail size should be 1280 x 720 pixels.
By Google’s own recommendations, your image needs to have a ratio of 16:9 and a minimum width of 640 pixels.
This ratio and size are most often used in YouTube players and previews.
While the thumbnail will be small when showing up in searches, it will be expanded to a full video’s size when it shows up as a suggestion for the next watch. For this reason, you want a larger image size that can be scaled down, instead of a small size that will be scaled up.
Some other important technical details include: