I researched these for my daughter, reluctantly deciding we didn't have space. According to what I read, axolotls will grow to 30-35cm (12 inches or more). They are cute, but we didn't have space for a big enough tank - I've seen recommended a minimum of 2 ft for a single axolotl and 3 ft for two. They need a cooler tank (14 C), and gravel is not recommended as a substrate as they can swallow it which isn't good for them. They don't like too much light, and need a dark hiding place (cave, pipe etc.) They may eat other small tank-mates, and other fish may attack their gills (even snails apparently!) so best to keep them on their own.
Hope this helps.