Working small freely enables you to think about the very basic composition re: lights and darks, shapes and placement of major forms. That is what was taught 'Old School'. Just using a 2B pencil or simple black wash you build a visual fluidity and eventually a rapid understanding of exactly what you are looking at.
Somebody said once "The sketch promises everything and delivers nothing "--- not so fast - It delivers plenty.
Here are a few small thumbnails by Mucha done on toned paper with a little pastel for some of his murals
|These were done as background studies for a sci fi TV film proposal for a Hasbro toy|