This is the rough drawing I did for what became "Daybreak Archer" for Magic: The Gathering. It was drawn smaller than I usually do when I'm trying to work out a composition for a painting but sometimes that helps. If things aren't working right in the drawing phase, I'll switch sizes to get a different look. Usually I go down in size since the images for Magic are only a couple inches wide in the end product anyway. Plus, going down in size will simplify shapes and it ends up making it easier to piece together something that works. That's what happened here (though this isn't quite "thumbnail" in size - it's about 6 inches wide I think).
On occasion I will ditch the thumbnails and jump into drawing out an image at full size or nearly full size. For me sometimes the little rough thumbnails aren't helping and I need bigger pieces and some important details put in before I can "see" where it's going. When that happens, it is usually because I feel a certain facial expression (or a similar "mood" detail) is the key to the whole image. If I can just nail down the right face, I can piece in everything else.
What's that you ask? Was there a background for this piece or was the mysterious bow-gunner just on a field of white? She's in a forest - the trees were added much later in the process and so never were part of the original drawing.