The best size pots to use for terrestrial orchids should be between a minimum of 125mm (5 inches) to a maximum of 175 mm (7 inches). Black plastic pots are better than terracotta as they last longer and are easy to sterilize.
If the pots are too small, they dry out too quickly in Autumn and Spring.
If they are too large, it gets too wet for the tubers. If using a larger pot, then the drainage needs adjusting by adding coarser sand to the mix. Specimen pots for show purposes utilize pot sizes up to 300mm holding 30-50 plants. Clay pans are sometimes used for show work.
The pots should not be shallow. 125mm depth is the minimum recommended for reliable results. For instance, the Arachnorchis like deeper pots as the tubers tend to go down deep. Droppers have been known to come out of the bottom drainage holes. When this happens to stop the tubers shrivelling up in summer, it will be necessary to stand the pot on the sandy soil of another pot.
Used pots can be cleaned by blasting off loose grime with a high pressure jet of water and then soaking in a bucket of water & White King (bleach).