We used 5×8 spiral bound sketch pad. The pages are thicker so they can draw, paint, color, markers and the color does not bleed through. I purchased them at Office Max last year.
I like the size, not too big, and they have plenty of pages. We used about 1/2 last year so they will continue with the same book this year. It is so fun to see their progression in skill.
If they draw smething specific, I write the description on the back of the page, but other than that it is their book to draw what they see.