I have 2 in a 60cm tank (60x30x30) = +-58L
They are now arround 13-15 CM in length and they are doing perfect

they both have their space and they are never curled up or cramped.
I am "hoping" they don't grow past 25 cm (max 30cm) so they can use 2 times the width and 1 time the depth of the tank and never come short of space.
But remember, putting something in a small space does NOT make the animal adapt in size, thats a fairytail
the only thing that affects size is food / waterquality and overall happyness of your axolotl
With this in mind, you must remember they can grow up to 40 - 45cm, and your axies will thank you for a bigger tank once they get past 30cm
On the other hand, 25-30cm would be the captive avarage i believe

so don't worry to much
Just make sure you have a nice filter, one that's designed for water volumes from 50-100L
If it has a dimmer, don't dim it, make the waterflow outlet push up against the glass and put some obstacles to block water flow at the bottom.
Get a nice nitrate cycle going and they are going to be just fine

Remember: Axolotl poo = ammonia > bacteria make amonia into fosfates / nitrite > bacteria make the nitrite intro nitrate and that's the final product of the cycle, and that's why you change a part of your water every week, to get the nitrate out and under 50 (ppl?)(ppm?)
The fosfates are used by plants, so live plants is a big YES
Also when the bacteria have no more "food" from the amonia they tend to die and leave an oily substance on your watersurface, this can happen and it just means there's no ammonia and nitrite available, and chances are at that moment there's only nitrate

It will disappear once there's food again for the bacteria