YouTube Thumbnail size: 1280 x 720 pixels.

1280 by 720 pixels is the 16:9 thumbnail size most people export and it still works. YouTube's own help page now recommends something larger: 3840 by 2160 for videos and 2160 by 3840 for Shorts, with a minimum width of 640 pixels, JPG or PNG, and a file cap of 2 MB uploading from a phone or 50 MB from a computer. The extra resolution matters on a television; on a phone the display is identical.

Thumbnails are judged small. In the mobile home feed the image is a few hundred pixels wide and in the suggested list beside a video it is smaller still, so a design that reads at 3840 pixels is decided at a fraction of that. One face or one object, 3 or 4 words at most, and hard contrast. The duration badge sits in the lower right corner and the red progress bar runs along the bottom of a video you have started, so keep type clear of that strip.

A 3840-pixel JPG squeezed under 2 MB needs quality around 80, which is fine for photographs and rough on flat graphics; for type and solid color, a PNG at 1280 by 720 or 1920 by 1080 stays sharp and small. The common mistake is exporting a portrait post and letting YouTube pad it. Thumbnails are always 16:9 and anything else gets black bars.