Dyakia hendersoniana
 

Hi everyone:
I've this orchid for awhile and its just not doing much. It is, however, overpotted (probably). What is the best practice in choosing a pot size? Is it better to under or overpot if I error?

The primary problems I have found with overpotting are two-fold:

In a larger pot, the center of the potting medium - right in the middle of the root system - can stay wet longer, so tends to break down faster than elsewhere in the pot. So, to the eye, the medium looks good, while it may actually be damaging the roots.

Plants seem to like to be mechanically secure in a pot. Wobbly plants don't grow as well as those that are well anchored by their roots. With a bigger pot, it will take the plant longer to become "comfortable" and take off with growth and blooming.

i agree with Ray in academic and practical applications....i do not adhere to that though.

i do not know the plant you are asking about but in almost all my orchids i have them in larger than necessary pots as i don't like to repot...i do not use organic medium that can break down though so it is a different animal.

there are a few plants that like to grow roots and if they are in big pots you may be waiting a while before they feel ready to flower in their big pot.

if you are using an organic medium then follow Ray's words

I totally agree with Ray. Overpotted plants in bark or sphagnum stay wet and airless in the middle. If one lives in a climate where totally inorganic medium such as volcanic rock work, the plant is really growing more like a mounted plant - plenty of air, water drains away.

Dyackia likes to stay moist with the highest humidity possible.