I have groovy boards for my hq16. They are about 8 inches I think but haven't really measured. I don't use them a lot. The pattern size has to match what you would like to do for the quilt. They take practice to use. I am thinking if you are going to trace anyway practicing and using pantographs are a better idea as they come in more sizes easier to store and cheaper to buy.
You loose approx 4" due to rails etc on most setups.