Moss is good; Ivy can easily be trimmed to keep it small. By "hotbox," do you mean a heated tank? If so, you may want to consider turning off the heating mechanism, as most plants probably won't need it. I believe certain other species of Fittonia will stay smaller than your polka-dot plant, if you wanted to look into that.
-Jmcat

Hello and thanks for your response.

No, this tank doesn't come with a heater, but it constantly stays around 77+degrees no matter what the room temp is, which is why I refer to it as a hotbox. I was under the impression that fittonia actually grew larger than the polka-dots, but I will look into smaller varieties of those, as well. I suppose I can find moss by just walking outside, and ivy is pretty easy to find, as well. Thanks for the tips :)

Hi
keeping plants in scale is one of the most difficult parts of terrariums the other is temps. I maintain my shadehouse as a "terrarium" 12x25 feet and everything still outgrows it .lol
What you need are "tropical" plants so be sure to use tropical mosses as temperates will waste away without a cool down.
Among the best are gesneriads, selaginellas. Which love the heat and can be kept rather small. Sinningias will flower under two inches in diameter. Some others are tropical sundews though they clump but easily removed. gary

One more question,

Since I seem to be having trouble finding generiads, selaginellas, or sinningias, (aside from african violets) can anyone recommend a place where I can buy seed(lings) of these plants? TIA.

Try the violet barn

There are at least 4 very small easy to flower sinningias. I had a seed sorn group of V. pusilla and white sprite on moss. over A.V. soil in a piece of L iliac root in my Terrarium It was pretty easy and splendid in appearance.

Have a great fall everyone1 Lee Mcdonald

In addition to those ideas already mentioned, there is also a type of miniature philodendrum/epipremnum that gets no more than a few inches tall. Mine reminds me of a mini "rubber tree". I can't say I've seen them for sale very often however. Might be because they are, IME, very slow growing. I've sold off divisions of mine now and then and have found that it can take a few years before a clump gets big enough for me to divide.

Finding plants that won't outgrow a tank that size will be a challenge.

I grow Anubias nana petite emmersed occasionally. It is VERY slow growing, survives low light, and as long as the humidity is high enough, does very well.

I've had a number of terrariums and one of the best, most interesting plants are the carnivourous plants. Mind you these plants do not need to be fed bugs. My favorite nursery for mailorder carnivourous plants is Sarracenia Northwest. They group the plants as to their needs and have lots and lots of care information so you can pick the right plants for your environment.

Here is a link that might be useful: Sarracenia Northwest

Yes, mexican Butterworts like the p. Moranensis and say a Sundew like d. Adelae or d. Capensis for examples would be good choices. Fun to grow! D. Adelae grows like a weed for me!