We had an escape wedding with a lunch served at the Attic. At the time that we had ours though (a little over a year ago) we could add guests for an additional $500 so we had about 21 people.
It was a a perfect size for us, pretty roomy, but it was such a nice day most of our guests sat outside to enjoy the sunshine and warm weather. And the view was great.
My top two pieces of advice- warn people how far they have to walk to get to the attic, and spring for using Disney's speaker system. We used an Ipod and docking station that we brough with us, but the music didn't reach to the outside portion when we had it hooked up inside and vice versa.