Traditionally, some categories are divided into small, medium or large.
Many items do not easily fit into just three categories.
My local store has four sizes of zip-lock bag, and labelling one size extra-small or extra-large just seems to cause confusion.
For things that need five divisions, I suggest that the categories of “small‑medium” and “medium‑large” be added to either side of “medium”. The structure of these terms make it obvious that they form part of a sequence of greater than three.
Optionally, the sizes of “extra/very-small” and “extra/very-large” may be added to the ends.
Having a five‑point or seven‑point graduation may be useful in using Likert Scales.
This system also may be applied to other progressions such as low‑high, light‑heavy or light‑dark, etc.
This may be cases where four sizes are needed rather than three, five or seven. Specific names for some of these sizes would make it obvious they are from a set of four other than three, five or seven.
For this application, the sequence I propose is: small, large-small, small-large, large.