Well size of tank is pretty debatable. IMO A 300g with the demensions of 96" long, 30" width/depth, and 25" tall. This would be good for life. Although once the aro was over 30" long you may find a larger tank is needed.
If you want to do the "grow out" method which is where you keep moving to a larger tank as it grows. Just always keep the aro in a tank that has a length 3 times that of the aro. The depth/width should be at least equal to that of the aro. Also take into consideration they grow quickly.