Repotting-what is the next size up

[fotofashion]

I have recently bought several oncidium intergeneric hybrids, flowering size, marked way down, probably needing repotting. They are all in square pots that measure approximately 3" square. Not all of them are overrunning the pot yet but several are. What is the next size up they should be potted in. And should I leave the ones that are not overrunning the pot in the same size pot but just give them new potting media? A bit of a problem is that the plants are so tall that they tend to fall over easily and I would rather have them potted so they are as stable as possible.
Does the advice about repotting when new roots are just starting to show apply to this species also?
Thanks for any advice and please point me to any other like discussions already held.
Beverly A.