I would say that it depends on the diameter of your tank and if it's a trailer mount or stationary. If you put a narrow FB on a wide tank it can be a pain to fit and it looks strange IMO. Here's a link to the calculator if you haven't found it http://bbqpitcalculator.com/ or we'll be glad to help you out if you give us a little more info.
Welcome.... You have come to the right place ! There are many very talented people here to help! The are calculator's within this site that will give info your looking for. Enjoy your build[emoji482]
Yep,
For the cubic inches and tuning number use the pitcalc.
I like to start with the diameter of the tank or so for the first dimension. the second dimension I do is height. for trailers cooked on from the ground using spring hangers I recommend no taller than 24 inches to keep the ground clearance. then manipulate the 3rd number to get to the desired tune. we typically recommend between 100% and 107%
Hope that helps!!