Is this the right pot size for my Dendrobium?
 

I have a few pictures here of my Dendrobium before and after my repot. I put it back into lava rock with some charcoal mixed in, since it did very well before. It was last repotted about 1.5 years ago but it grew 5 canes, two of which are growing now. It was climbing out of the pot it was in and I put it into a 5' pot. I think it is one or two sizes up but the pot seems big to me. The Dendrobium has a GREAT root system and I would not want to rot it out. I don't really have any other potting option my other pots are the same size as the one it came out of. Hopefully this is right!

works for me, looks very good!

The problem with Dendrobiums like that is they grow best in pots that are simply too small to hold them upright. Extremely frustrating! I personally would have moved it into something only slightly bigger than the original but it should be fine in the one you used. It isn't oversized enough that it would cause problems.