When it comes to the tank an aro could fit, it also matters on the type and breed you're keeping.
For silvers they could easily max out at about 3'. So the no1 consideration would be the width of your tank. Idealy should not be less then 3'. The length should not be less then 5'. But larger tank is always better.
If you're keeping asians aro....then again depending on the breed. Gold varieties usually max out at about 24" could be slightly bigger. So the tank width should not be less then 2' and the length should not be less then 4'.
Red asian aros have the same requirements as silvers due to size it could grow to.
You could keep more then 1 aro in a tank sized above but you'll need to be hardworking in doing water changes.
Anyone have any differing views? Would glad to hear what you have to say.