I’ve been in the habit of carrying around a small spiral-bound notebook (about 5″ x 7″) for jotting down ideas. I used it a little bit for sketching out screen designs for mobile apps, but I had to consciously think about the size of a mobile screen compared to the size of the page on which I was drawing. However, I’ve recently found that a slightly smaller notepad is just about the perfect size for these UI mockups.
The size of a 3″ x 5″ notepad is pretty close to the size of a mobile phone, which allows you to get a quick feel for whether or not screen elements are sized appropriately. You can also hold the notepad in one hand to check the feasibility of single-handed use. The notepad is also small enough to carry in a pocket to quickly experiment with UI layouts while waiting in line at the store or other places around town.
If you’ve found a cool way to experiment with UI layouts, go ahead and share it in the comments.