tips using mark up in iMac preview.

FWIW, you can't reduce the size of the "eye dropper". However, there is a one pixel square in its center to pick a color with. As far as removing the color samples, click on one and press delete.

While you cannot reduce the size of the dropper, you can increase the size of the image. That effectively accomplishes the same thing.

It works in Monterey. Are you actually using Catalina? If so, I can check a Mac I have that is still running Catalina.

Try double-clicking on a color patch to make sure it's selected.

FWIW, the area of the eye dropper acts as a magnifying glass so you can pick out individual pixels.

No problem.