Quick answer
The standard YouTube thumbnail size is 1280 x 720 pixels in a 16:9 ratio. Design at that size, keep text short, and leave space around faces and key words so the idea still reads on a phone.
Start with 1280 x 720 for YouTube
A 16:9 thumbnail is the safest default for regular YouTube videos. It fits watch pages, search results, embeds, channel pages, and recommendation surfaces without forcing you to redesign the same idea.
Treat mobile as the real test
A thumbnail that looks good at full size can fail when it is squeezed into a small feed card. Put the main subject and the key text in the central reading area, then check the design at phone size before exporting.
Do not solve every format on one canvas
Use 16:9 for normal YouTube covers. Use 9:16 only when you are designing a vertical cover for Shorts-style surfaces. A square 1:1 version can be useful for social previews, but it is not the main YouTube thumbnail format.
Rough idea
My video explains why my first 100 thumbnails failed
100 FAILED THUMBNAILS
Large text and one clear proof pile stay readable in the standard YouTube frame.
TOO SMALL
This direction makes the phone-size problem visible instead of explaining it.
FIX THIS FIRST
A split layout teaches the sizing lesson while still creating a clickable promise.
YouTube thumbnail size checklist
FAQ
What is the best YouTube thumbnail size?
Use 1280 x 720 pixels with a 16:9 aspect ratio for standard YouTube videos. It is the safest working size for most creator workflows.
Should I make YouTube thumbnails in 9:16?
Use 9:16 for vertical Shorts-style covers or social reuse. For regular YouTube videos, keep the main thumbnail in 16:9.