A three foot(90cm) tank with the largest possible floor space would be sufficient for two. Do you know the sex of them? If they are male and female you would be better off buying a very large tank like 4 foot(120cm) and dividing it in half or have them in separate tanks.
By the sounds of it your axies are already big enough to be moved to a bigger tank.
You will need to cycle the new tank before you axies go in it. Alternativly if the tank you already have is cycled you can transfer the filters, substrate, and ornaments over to the new tank and then top it up with new water you will be half there.
ADDED INFORMATION: I just saw you are not to sure about what sex they are. To help with this here is a ROUGH guide. Females have a rounder head and are plumper. Males have a slimmer appearance and have a slight buldge just behind the back legs which is called a cloaca.
I hope this helps.
Blaze