Doing thumbnails can help you work out both the layout and make sure you understand the values you want to work into your artwork.
Thumbnails, to be effective should be done in the same aspect ratio as the finished work.
So decide how big it will be.
We are working on 5x7 still lifes. Our thumbnails were worked 2 1/2 x 3 1/2, which is the same height to width ratio.
Each work will be done on different surfaces, and has a different viewpoint.