There are some pretty small Microgramma. They did real well for me in a tererium(sp) They've proven tricky outside. The one I used had circular leaves/fronds are about 3/16" across, long creaping rhizomes.

I like ebony spleenwort.

Maidenhairs such as Adiantum capillus vernis, venustum, and hispidulum, and the small deer ferns: Blechnum penna-marina types. These are hardy for me but might be good terrarium candidates for you. Any good fern nursery, like the one linked below, will give you mature sizes.

Here is a link that might be useful: Fern list at FTF