No packer briskets in a Davy... that's it's main downfall...
As RemoGaggi was saying, while the internal dimensions of the Davy are just shy of 12.75x17.75", you really have to reduce the front to back dimension by at least 3 inches, if you want to try and avoid the worst of the heat that rolls up around the edges of the drip tray... So, your probably really looking at closer to 9x16" of space that's mostly void of any excessive heat... Having said that, I have cooked two racks of ribs on the main grate... you just have to be aware and understand that extra heat around the front/back.