GUIDE / THUMBPULL
YouTube Shorts Thumbnail Size
Shorts use a vertical thumbnail workflow, so a wide 16:9 thumbnail should not simply be squeezed into the same frame.
Use a vertical composition
YouTube's current help documentation recommends 2160 × 3840 for Shorts custom thumbnails with a 9:16 aspect ratio. The important practical difference is composition: faces, text and logos need to work vertically rather than being recycled from a wide video thumbnail.
Use the Shorts preset as a quick working size, or enter the current recommended high-resolution dimensions manually. Fill, Fit and Crop let you decide how a horizontal source is adapted without accidental stretching.