You dont want to go too long with a stack, it changes the airflow of the pit. Using a 90 will slow the air up a little bit, putting a full stack on will draw it out of the pit a little differently. What are you wanting to add the stack for? What is the application?
It is 5", and you can get the parts at Home Depot or Lowes, or the hardware store. We installed a 90* elbow, and a 30 stack with a rain proof cap. Use it every cook. I think the stack/cap keeps the wind from getting inside and messing with the temps!
I think he meant 30". I would go with a stack to the top of the pit, that includes whatever you would use for a rain cap. A 90 with no cap works great. you just turn it downward facing if it rains too much...
To to plumbing supply and get a 5" combination elbow. It has one end that is crimped down to fit right inside the FEC outlet the combo part lets you move around to flow downwind. It should cost less than $5 for the piece.