As far as I have been able to find, there are no "hard and fast" rules for a mature plant. Many times it is dependent upon the genus and species of the plant in regards to what constitutes for "mature".
A good rule of thumb to follow is if the plant is of blooming size/has bloomed, it is "mature" (at least for this scenario for the home grower- not necessarily biologically). I personally vary my phal sizes to have variety (anywhere from a 2.5" mini phal to a 6" pot). It really depends upon the space that you have and what you are looking for in your collection. But if the plant has bloomed, I would say that you are good to go
P.S. I warn you now- once you get some orchids that you love and are able to keep alive, you will always want more!