What type of lighting are you using? I currently have an unknown Oncidium growing in my terrarium that is about to bloom. It would fit your criterion if your lighting is strong enough. Oncidiums are pretty easy to find--this one came from Lowe's in the clearance section for $1 (but was marked a Paph).
I would stay away from Phalaeonopsis orchids. While common, they are not free flowering, and require a 30 day period where the night temps get down in the 60's to encourage bloom (anyone repeatedly flowering phal's in a terraria with darts, please correct me if I am wrong).
I have had very good success with Dendrobium hybrids (Emma White and other hybrids in particular), but those get 18" canes, so they are too tall for your specifications.
I have several mini Cattleya hybrids that are still not blooming size, but thriving in terraria--those would definitely fit the bill. I could give you the contact info for the grower I got my minicatts from . . . but I'm not sure he still has any.
Haerella odorata (or retrocalla) is another good mini for the terraria, but much smaller than the others listed here. Some of the Pleurothallid alliance plants will also do well, but very few have large blooms.
Paph's will do well in the terraria, but are not generally good candidates for epiphytic mounting.
Oh well, I'll stop rambling and let you determine if I have given you any reasonable info.
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Homer W. Faucett III, esq.
Purveyor of Trivialities and Fine Nonsense