Sounds good to me. I would do three 8" squares with 3" sashing between them and a 3" border all the way around. That makes it nice and balanced. It would be 36" by 14". Or, better yet, increase the border to 4" and have it 38" by 16".
Play with different measurements on some graph paper and you should be able to tell what looks good to you pretty quickly. You can even find graph paper on the internet (or use Excel like I do). :mrgreen: