I've had both schilleriana and the bastanii mounted. The bastanii foliage and roots don't get overly big but the spikes when mounted grow sideways so the width can be an issue.
The schilleriana I mounted on a 6" wide by 12" long mount. The roots grew several feet below the bottom of the mount. The spikes grow up, then start arching down. Eventually the spike will be 4' long. I came into the g/h one morning and there, laying on my table was the spike. The weight snapped it off. The next year I had to support the spike by pushing a wire through the mount. IMHO this is a hard Phal to mount.
Katrina gave you a great list of species/small growing Phals. Her suggestion of some of the Angraecum family is good but most of them have short lived blooms.
Phal schilleriana 'Highjack' AM-AOS LUR_5704 by
Jim Lurton, on Flickr
Brooke