In short
- Point at the part you mean with a box, not a sentence.
- Judge it small, next to its title, on a phone, in dark and light.
- Compare versions side by side.
- Keep the references next to the image.
Point at the part you mean
“Make the face bigger” is clear only if there is one face. Drag a box over the part you mean and write the note there; the designer clicks the note and the box lights up.
Judge it at the size people see it
A thumbnail is seen small, next to a title, in a feed full of other thumbnails, often on a phone in dark mode. Review it like that, not full screen: text that reads at 1280 pixels wide can vanish at feed size.
Compare versions side by side
Upload the next version onto the same thumbnail and look at them together. It is much easier to say which one works than to describe what is wrong with one on its own.
Keep references next to it
Lay the reference images (a shot from the video, a competitor's thumbnail, a colour) on a board around the thumbnail, so feedback and inspiration sit in one place.
Questions
What size should I review a thumbnail at?
At the sizes people see it: in a feed next to other thumbnails, beside a video, and on a phone. Full screen hides the problems that matter most, like text too small to read.