It probably depends to some extent on the genus you plan to repot but in general one should use a pot large enough to accommodate one or two year’s extra growth but no more. I fully agree with Alfred Smollet, who in 1927 wrote in The Orchid Review “I am a great believer in small pots. A common error with beginners is using pots unreasonably large. All beginners in Orchid growing are optimists, and the large pot is the outward and visible sign of this optimism”. One of the main problems in using overly large pots is that the potting mix tends to remain wet for too long after watering, especially in winter.
The choice of pot size is largely a matter of common sense. When the orchid is knocked from its pot, cursory examination will show that, in most cases, the majority of its roots run around the circumference of the pot in preference to filling the pot completely. And if you examine their condition you will generally find that those growing around the perimeter are in a much healthier state than those in the centre of the mix. Dome® pots are designed with a dome in their bottom to eliminate the area most prone to root decay. An alternative is to place a small, inverted pot in the base of a larger one before repotting. Cymbidiums roots of a healthy plant generally fill the entire pot and remain in good condition for a couple of years. But thereafter, as the bark begins to decay, the roots in the centre of the pot are usually the first to deteriorate.
It’s also important when repotting to ensure that the drainage holes in the pot are unblocked. Plastic pots made using old dies (moulds) sometimes have the drainage holes partly or fully blocked with plastic. It’s important to open the drainage holes fully, using a lino knife or similar sharp implement. Sometimes I enlarge the holes at the same time to provide extra drainage. Some growers use an electric drill or hot soldering iron to make extra drainage holes in their cymbidium pots and swear that it results in faster growth.
Pots are produced in a variety of shapes and heights, some with far more drainage holes than others. Best of all, in my opinion, are Port Pots®, which have an extensive network of drainage holes in their bottom. In general, shallow pots seem to suit Australian native dendrobiums best. Net pots, designed to provide excellent drainage, certainly do that but they dry out quickly and therefore need to be watered twice as often as pots with fewer drainage holes. Oncidium seedlings do well for me in them but the roots tend to grow in and out through the mesh holes. This leads to problems at repotting time – either one cuts off the offending roots or one simply puts the whole pot into a larger one and tops up with new mix; each procedure has its disadvantages.
The above discussion refers to plastic pots, by far the most common in general use. But the older pots made of baked clay (terra cotta) are useful in certain cases. Because they are porous, water evaporates from their exterior, producing a cooling effect, which can be beneficial in hot weather. However, this is a disadvantage in winter, when it’s better to keep the roots warm. Some growers prefer clay pots or saucers for temperamental orchids, such as Dendrobium cuthbertsonii and Sarcochilus ceciliae, which need a reliable supply of water but hate ‘wet feet’. But in general I believe that black plastic pots are better, especially in winter when they help to warm the roots whenever the sun shines on them.