I was a little confused there about the dimensions too. I have a BYO 11'6" X 15'6" and it's very roomy. 6' X 11'6" is a warm room and the rest observatory. I would always build as big as you can keeping in mind your budget or space or any other requirements such as HOA or the like. Building it yourself makes building bigger cheaper too and you may look at the cost of materials as not that much compared to what you get in the end.
I don't have a dob, but I would think if you are using a dob in an observatory you would want it on the larger side.