Should there be a specific size to the cage? What else besides rocks?

Found this great site for making a Salamander habitat.
There is a video on the site on a guy showing/telling you what the best habitat is and what to use.

Alternatively, if you don't want to watch the video, there is a transcript further down the page:

How to Make a Habitat for Tiger Salamanders: How to Care for a Pet Tiger Salamander | eHow.com