I assume that since they came "stuffed in the small plastic pot," the current media may be sphagnum moss. An important factor on if and when they should be repotted, as well as pot size, is the condition of the roots. If the roots are healthy and wrapped tightly around the inside of the pot, you could go to a slightly larger pot that will accommodate the roots. If the roots are rotten and/or dried up, cut out the dead ones, and stay in same size pot or smaller. (again based on fitting pot to roots) You might try an orchid bark mix (bark, sponge rock, charcoal) or stay with sphagnum moss. The bark mix may be better in your case. Don't overwater! By the way, there is a lot of info on phalaenopsis culture, including repotting, potting mixes etc. available on line.