Shorts thumbnails are vertical: 1080×1920 pixels, a 9:16 frame matching the video itself. Design anything smaller and YouTube upscales it into mush; anything with different proportions gets cropped unpredictably.
The bigger trap is the safe zones. The Shorts feed overlays UI on top of your thumbnail: the title, channel name, view count and action buttons sit over the bottom fifth, and the account bar hugs the edges. Put your hook there and it disappears.
The practical rule we build into every template: keep the top 150px and the bottom 420px quiet, and let your hook live in the middle band between roughly 300px and 1400px from the top. Toggle the safe-zone guide in the editor toolbar to see both zones live on your canvas.
Text size follows from feed reality: thumbnails render about 200px tall in most feeds. A 150px headline on your canvas is about 30px on a phone — readable. A 60px headline is a smear.
Export at full size. The standard export is exactly 1080×1920, as PNG or JPEG — no compression surprises. Pro users can export 2K or 4K renders of the same design for crisper text on high-DPI screens, but the standard size is what YouTube wants.