About YouTube Thumbnail Size
A YouTube thumbnail is 1280×720 pixels, a 16:9 ratio, and must be under 2 MB. That size is the upload specification, not the display size — most viewers first see your thumbnail at somewhere around 360×202 in a sidebar or on a phone, which is roughly a twelfth of the area you designed on.
That gap is the single most common thumbnail mistake. A subtitle that reads clearly at full size becomes an illegible smear at browse size. Three or four large words is usually the ceiling, and faces work because they remain recognisable when small.
The channel banner is the other trap. You upload 2560×1440, but only a central 1546×423 region is guaranteed visible — televisions show the whole thing, desktops show a wide strip, and phones show that small central band. Anything outside it is cropped away without warning.
How to size a YouTube thumbnail
Export at 1280×720
16:9 exactly. Larger multiples of the same ratio are fine; smaller images get upscaled and look soft.
Keep it under 2 MB
YouTube rejects larger files outright. Compress it if a detailed image pushes past the cap.
Check it small
Shrink your design to around 360×202 and look at it. If the text is unreadable there, it is unreadable where it counts.