On the vertical smoker, the calculations for the openings are not as much as a factor as long as you can achieve enough air flow to feed the fire and have a big enough exhaust to keep from choking down the air flow.
DaveOmark made a good remark recently when giving some advise for a vertical smoker, something along the lines that they are more for commercial application and if insulated can be an efficient smoker. I believe he hit it on the mark with that statement. The key to a successful vertical smoker is for it to be well insulated where as you really dont need a big fire in order to create enough cooking heat inside the chamber, with the smaller fire allowing the heat differential to be more manageable. Most commercial operations will load these with uniform cuts of meat, so there is not much opening of the chamber going on during the cooking process,.
If that sounds like the style of cooking your wanting to do, a vertical smoker should work well. I have had great success with them doing ribs, and smoking mullet.
But if your looking to just do some entertaining, and cook different size cuts of meat at the same time and tend to like to open the door often to play around with the food while its cooking, then I would recommend that you take a second look at an off set style smoker.
Anyway, good luck and happy smoking!