What size does a YouTube thumbnail appear in search?
Around 360 by 202 pixels on desktop search results, which is roughly a quarter of the width of the 1280 by 720 file you uploaded. YouTube only scales your image down, never up, so every decision in the design has to survive that shrink. Upload at full 1280 by 720 anyway, because YouTube generates all the smaller versions from it and a small source just looks soft. The exact dimensions for every surface are in the YouTube thumbnail size guide.
How small does a YouTube thumbnail get?
The smallest common slot is the desktop up next sidebar, roughly 168 by 94 pixels. At that width a face is about a fingernail wide and text longer than three or four short words turns into grey texture. That preview at the top of this thumbnail tester is the real test. A design that only works at full size is a design most viewers never see working.
What is the safe zone on a YouTube thumbnail?
The safe zone is the part of the image nothing else covers. YouTube stamps a duration badge over the bottom right corner, and once someone has started watching, a red progress bar runs along the bottom edge. Keep faces, logos and the end of your text out of that corner and off the bottom few percent. The 6 percent dashed margin in the overlay is a working habit rather than an official rule, and it buys you clearance from rounded corners and cropping across surfaces.
Why test a thumbnail in greyscale?
Because greyscale strips colour and leaves only brightness contrast, which is what the eye resolves first and what still works at 168 pixels or on a phone with the brightness turned down. If your subject melts into the background in the greyscale preview, colour was doing all the work, and colour is the first thing to fail in a fast scroll. The usual fix is a darker background behind a brighter subject, or a hard outline around the subject.
Want to test someone else's thumbnail instead of your own, to see what you are up against? Pull the full resolution file with the YouTube thumbnail grabber and drop it into the tester above.