i have been researching heavily over the past few months as I prepare to get my own and this is what i've come to learn as a common denominator for success for the AT from other reefers (Disclaimer: i havent kept one myself!)
- The longer the tank, the better, they will use every inch of it, lapping around the tank constantly
- High energy usage on the Tang's part will require high amount of feeding as well, extra water volume, higher than stated turn over volume on skimmer etc will help deal with the increased feeding/bioload
- Really high, turbulent flow as they live in surf zones
TL;DR, i think 5-6ft in length for a tank should be good for a small-medium AT for a good few years. I'd only be concerned about gallons in terms of absolute water volume to deal with how much feeding the AT will need.
Let us know how it goes!