Hi Jadore,
To provide the best environment and space for the axie, i would say 2 1/2 foot is alright for a full grown adult, but preferably not 2. I suppose it depends on how you wish to look at it. If you want to provide adequate space to include 2 hiding place per axie, 2 1/2 foot tank might be a tight squeeze for a pair. If you intend to have a no frills tank, it probably can still hold 2 but in much less than ideal conditions.
Ultimately its still the dimensions that matter the most so if you have a tank with a huge surface area but short, i suppose its still workable.
In general, its best to get the biggest tank you can afford because the bigger the tank size, the easier to manage the tank.
Cheers