R L,
It's really all up to you. You may, with your growing family, opt for a larger TD. I don't think it'll cost too much more in the long run to build a bigger TD. Another thing to remember, that long before you're finished with the TD it will be "Campalbe" if there is such a word. You know what I mean. The interior might not be complete, but you certainly can use it for camping.
I hope this helps.
Steve