I would suggest something around 5-6' long, and 1.5-2" wide. As long as you closely monitor your water parameters and do 1-2 water changes weekly, that should suffice. If you can't find what you're looking for (don't forget to check kijiji and craiglist) see if there's someone who custom builds tanks.
Make sure you have ~2 hides/axolotl to help prevent accidental nipping and provide security!
Adding faster growing plants (bacopa,cabomba,egeria densa,wisteria, hornworts, water sprite, duckweeds, salvinia, water hycinath) will use up more nitrates in the water and will thus help keep nitrate parameters clearer. Most of those are floating plants - since they are exposed to the air they can access greater amounts of CO2 and thus photosynthesize more, ergo using up more nitrates! Floating plants also second to decrease light in the tank, and since axolotls don't have eyelids, they appreciate not having bright lights shone in their faces.
You could get 2 tank instead, if that size of a tank would be too big, however that decreases the footprint of what each axolotl would be able to maneuver about, so keep it 1 if you can and/or you don't mind keeping boys with girls!
