Okay, so this is a simplistice view on things, but:
An aro can easily exceed 24", so we will call it 25.
My tank is 3x3 which means it is 3/5ths of your tank size so by the same respect I could keep something that was 15".
Which I would not do, I know I'm going to feel bad when my GTs hit 6".
~
The fact of the matter is: you will do as you please, nobody here is going to stop you. Only you yourself will deem whether or not the tank is appropriately sized. If you buy it as a juvenile you may think when it hits 18" that its too big. At least then you've had some experience/time keeping an Aro. Or, you may decide that its fine and keep it in there for life, your choice.
But IMO once it exceeds 18-20" then you need to move it on.
Sure, people on here keep them in smaller tanks, but why make your own standards based on others. We all know you do a good job of taking care if your fish at current. Why ruin that reputation?