That sounds simple and cheap. Good idea! I found the directions to make this bit sizer and it sounded complicated and too big. There is a bit sizer you can buy for about $20. So, I didn't like either of those options. A pencil though, I think I can handle that one.
The pencil is your best bet! Stick it in the horses mouth with the eraser on the end sticking out, mark the other side with a marker and add 1/4 inch to the measurement. If you get an inbetween measurement go up to the next size not down.