You have to apply following css code into your current template css file.

div#fd.eb .eb-category-thumb .eb-avatar img {

    width: 100px !important;

    height: 100px !important;

}

I use yootheme template. This template has custom.css file, so i think can i insert that in this file, correct?
The meaning is that the avatar must have the above sizes?

Yes, you can apply into your template custom.css file.

The meaning is that the avatar must have the above sizes?

Nope, you can adjust these width/height pixel and see which size you like it.