When I'm trying to figure out a design idea, but I dont really think it's going to go anywhere or if I want to cycle through a lot of ideas quickly, I usually do 5 minute sketches like these. They're usually pretty small in terms of size, maybe about 3" x 4" maximum and sometimes even smaller. These are also referred to as thumbnail sketches. This style of sketching helps with quick ideas because it's much faster to create so you can prove out and move forward the idea your really want to work on.
Quick Sketches | Thumbnail Drawings
Thumbnail sketches are great for working out ideas quickly before getting into the more time consuming details of larger sketches and renderings.