I find this drill/process very helpful not only for my students but for me. I don't care what the subject matter is because it's all about interesting shapes and values. Almost all of these images were about 5 x 3 inches and I used a larger brush because I had to cover an area quickly. This teaches you how to paint big to small and to capture the essence of your concept. Also teaches time management, how to draw accurately and decision making skills. Doing this drill will get you to next lever faster and make you a better artist. BTW, take a pause or timeout for drying and you can use gouache if needed... I think you'll surprise yourself by how much you can get done in 10 minutes. Have fun!