ABOUT THE ARTWORK
When I know what my story is going to be, it’s time to start finding out how it will fit into the allotted pages of a book. This is when I do my storyboarding, quick sketches which get across the main point of the action. Some scenes demand more space than others - some need to happen quickly (big one page or even one spread size panels) and some fast (half or even third page size panels.)
Sheep’s caravan crashing into the lake is the main action here, and so in the final design you will notice that it got about three quarters of the spread (rather than half of it).
(From Books For Keeps)