Rosie:
You still have some time to think about what pot and media to use, as soon as a new growth appears its time to repot. Reed type dendrobiums grow almost anywhere and yes they tend to fall when in small pots. I had many 4" square pots with dend's and onc's and they kept falling so I made a 4' x 8 ' frame with pressure treated 2" x 4" s and placed on top a lattice panel whose holes are a perfect match for the pots (see pic).
Hope this helps you and others.
Jose