How big should a terrarium be for orchids?

[Optimist]

People do tend to forget that orchids grow, and they actually can grow quite large, even those with tiny leaves. With terrariums people also tend to want backgrounds, and they will need various environmental things, just like a fish tank needs a heater and a filter. With a terrarium you often need fans, misters, lights, possibly heaters or even coolers. With fish tanks they say the bigger the better, and that goes for terrariums as well.