I recommend at least a 8 foot long tank, but if your building a custom tank the 9 foot will be better. If 24" is as wide as you can go then so be it. I would try for at least 30", or more if you can. The biggest advantage will be the farther away from the from glass your aro can get the less skittish it will be. I saw a remarkable change in my two when moved from a 24" wide tank to a 36" wide tank. Plus if they do become spooked they have much more room to move.
A height of 24" will do as a minimum, going higher will greatly increase the cost of your tank as the glass thickness will have to increase. The advantage of having a taller tank is more room if you decide on a community. The added hight allows for more defined separate swimming levels. Also if you have a lot of aquascape or decor the short tank moves that up into the aros swimming area.