According to Feldon's 5.5" needs to be 82.64 " tall Way to tall, 2 - 5.5" about 42" still pretty tall, 500 gal is a pretty big smoker besides working properly you want it to look good, especially after you put hours of work into it. That was a good deal on the 5.5" It it were me I would keep checking and find some 8" One that size smoker it will look and work better. The problem with real long smoke stacks besides looking funny is cooling, As the smoke travels up a long stack it cools, cooler air is heaver and you don't get the flow that you really need, not saying it won't flow, just not as efficiently. Large smokers need to flow a lot of air, heat and smoke, you don't want stale smoke staying in your cook chamber. A 500 gal smoker needs 1,964 cu. in vol.. The problem is length you can get the same volume from 2" if it is 635" long . My opinion from the smokers we have built, we try to go with a stack that is proportional to the cook chamber, and as Dave said around 30 to 35" now that being said 8" would need to be 39", according to the calculator But when you get to that diameter the length is not as important, you can easily make it 30 to 35 " with no problem
Gary