My Axie had a 20 gallon long to himself...good job too cos he grew way over the "average" size.
You'll not be able to sex them til they are a year old, you might get hints beforehand but no definitive sexing til a year. If two females or two males they need to be put in different aquariums as with most things they are partial to biting limbs and gills etc off each other in fights. It takes around 2 to 3 months for said bitten off parts to regrow.
Sandy or bare bottom aquarium is a must. No stones/rocks/ornaments smaller than their head size cos they are hoover eaters and anything smaller than the head will end up in their digestive tract. If that happens (and in cases of overfeeding/constipation) you need to move your Axie to an aquarium water filled sandwich box, secure the lid and place in the fridge...yes...literally your fridge...for 5 to 10 days to allow the metabolism to slow enough to pass the object slowly and not damage the animal fatally.
Hiding places, NO lighting, NO heating, NO handling and definitely NO fish.
Phoenix, my Axie, lived a little over 2 years with me. The breeder suspects that his heart gave out due to growing so big....way bigger than normal. He would swim every day....I counted the laps a few times, 40 laps was his average daily swim distance. He would greet me in the morning with smiles, burps and farts and he ate like a horse. They are very playful and due to having lungs and gills they need to breach for air often. Make sure the lid of the aquarium is very securely fitted since when fully grown they will headbutt it and they can jump out if the water chemistry is wrong or they feel unsettled for any reason. Water chemistry needs to be taken care of just as any aquatic animal, they are seriously big poo machines, so turkey baster for poo collection, good filtration essential.