How big should a terrarium be for orchids?
 

'ello all! I wanted to ask...would a 5 gallon fish tank of 16 by 8 inches floorplan (and 10 inches high) be large enough for miniature orchids? And would a 24 by 12 inch 20 gallon high (with 16 inches of height) be large enough for draculas?

Miniatures vary in size a lot, but the 5 gallon would be big enough for many types. And you might have to restrict yourself to ones that don't get longer spikes. With smaller minis with short spikes you could get quite a few in there.

You could probably get a few Draculas in that 20 gallon high, but again spikes might be an issue. Some Dracs can have spikes that spread or hang below, taking up much more space when in bloom.

It becomes much easier to maintain stable temperature, air flow and humidity in a terrarium as the size increases. Cost and space are always an issue, but the biggest terrarium you can install will be easier for you in the long run.

Plants have this odd habit of growing. As estacion seca pointed out, environmental control gets easier with larger size. My smallest one was a 30 gal, now I have a 90 (63x14x24") and a 180 gal (36x24x60") tanks. In the 60" tall warm tank, I can grow some larger plants such as Malaxis s.l.
Draculas are on the larger side, so I would not go below 20-30 gal. Be very careful which species you select.

Ditto what the others have said; though we should also ask if there is anything specific you are looking to grow. You've stated Dracula, but they run the range of sizes (as most Pleurothallids do...); I'd suggest putting together a list of plants you think you'd like to grow, verify their availability and size a terrarium from there. You might also consider focusing on a single genus, ie Porroglossum for interesting plants when not in bloom, small size, interesting flowers; Sophronitis for challenge, bright flowers, floral form, etc.
Hope this helps a bit, please don't hesitate to ask questions; there's a lot of us here that grow many different things.
Adam

About the size of a greenhouse is best...

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.