YouTube thumbnail guide — 1280 x 720
from getyoupush.com/tools/background

What YouTube draws on top of your thumbnail:
  Duration badge        bottom-right, roughly 132 x 36 px with an 18px margin
  Resume progress bar   full width along the bottom ~10 px, on partly-watched videos only
  (YouTube tunes these insets; treat them as close, not exact.)

Safe box for the face and any text:
  x 40  y 40  1200 x 610

Sizing
  - A thumbnail is judged at about 210px wide in the sidebar and smaller on a
    phone. That is a 6x reduction from 1280px, so text below roughly 54px in
    your 1280x720 file lands under 9px where it is actually seen.
  - Check it by shrinking your own thumbnail to 210px wide and looking at it
    from arm's length. If you cannot read it, neither can anyone else.
  - A busy background is the usual reason a face stops reading at that size.
    Cutting the subject out and dropping a flat colour behind them fixes more
    thumbnails than any amount of text tweaking.