Generally speaking the more water the better, A larger volume of water is more stable quality and temperature wise than a smaller one. There is a bigger margin for error in a bigger tank, ammonia or nitrite can build up pretty quickly in a small one, especially if you're just starting to learn about aquarium keeping.
I would remove the reptile carpet or whatever it is, it will make cleaning the tank a nightmare and there are no benefits to it being in there. I sometimes use washed kids play sand if there are going to be plants that need a substrate, or just leave the bottom bare for easy maintenance.
In my opinion the most important parts of a tank for newly imported firebellies are excellent water quality, plenty of live plants and no water movement. By the time they reach petstores imported newts have often gone into terrestrial mode to escape the poor conditions, this means they lack the broad tail fins needed for swimming and can actually drown in open water, the masses of plants give them something to hang onto and the still water makes it easier for them if they do decide to swim. Once they start spending time in the water they will regrow their tail fins so the plants can be thinned out a bit if you want, but Firebellies are dependent on plants for hanging out in and egg laying so they should really have access to thick vegetation.
It goes without saying that a tight fitting lid is essential, firebellies can easily climb the glass or emergent plants and escape through some pretty small gaps. Unless they are found in a matter of hours they will dry out and die very quickly.
