When I think about the right size of Gravel for a tank, I think about a human trying to fit a Basket ball inside of their mouth. I think this should be the same way of thinking for an Axolotl. A really large rock or pebble and if it can fit into an Axolotls mouth. If it can't, then it is all good. Personally, I think sand is best because the bigger the gravel, the harder it is to clean. Sand is pretty low, easy cleaning. hope this helps ^_^