When it comes to painting, thumbnails aren't things you've got on your thumbs, but rather very small and rough sketches outlining the elements in a proposed painting. Thumbnail sketches are a good way to work through an idea, or to try out different compositions.
There are no rules on how to make a 'proper' thumbnail sketch. It's entirely up to you how big or small you make it, and what medium you use. Some artists stick to pen or pencil, others use colored pencils or paint. Some add brief annotation, some write detailed notes on on their thoughts or idea.
It's also up to you whether you start by creating a rectangle the shape (proportions) of the paper or canvas you intend using, before you create the thumbnail in this.
I usually make my thumbnail sketches about an inch (2.5cm) on the longest edge. The photo on the right shows a set of four (it's been reduced in size), done using black pen and watercolor pencils.