How it works
1280×720 with a 2 MB ceiling. The file size matters less than where it gets shown: in the sidebar and on phones the same thumbnail is drawn at around 210×118, a sixth of its size.
That is the real constraint. Text under roughly 30 px in the original becomes unreadable at sidebar size, and a face smaller than a quarter of the frame stops registering. Design it at the small size, then export at 1280.
Frequently asked questions
How small does the thumbnail actually get?+
Around 210×118 in sidebar suggestions and on phones — a sixth of the size you designed. That is where most impressions happen, so it is the size that decides whether it works.
How big should the text be?+
At least 30 px in the 1280 px original, and better around 60. Three or four words maximum: anything longer is unreadable before anyone clicks.
Does the 2 MB limit affect quality?+
Rarely — a 1280×720 JPEG is normally well under it. YouTube re-encodes to WebP anyway, and flat areas with strong contrast survive that better than fine detail.