I purchased the 6 3/4 x 10 1/2 and the 7 1/2 x 10 1/2 from 2 different comic book stores. I was happy with the way the 6 3/4 worked with the fabric. Besides the 7 1/2 size was almost twice as much as the 6 3/4" size. That made my decision easier.
Here's a link to a thread I started. It contains pictures and great opinions.
http://www.quiltingboard.com/mission...s-t179823.html
I think it was well worth the $20 I spent on the boards. I can see all my fabrics in 2 bookcases that I picked up at a garage sale for $2.50 each~!
Good luck