Q:

Is *-scule* in *minuscule* a suffix?

Is -scule  in minuscule a suffix? 
What does it mean? scale?
(I have looked it up in etymonline and didn't find the answer)

A:

The word minuscule refers originally to small letters, and comes from Latin minusculus, which, according to Merriam-Webster is a diminutive form of minor. It is analogous to majuscule, which is a diminutive form of major. That is, the Latin comparative adjective minor meaning "smaller" has a diminutive form of minusculus. If there is any "suffix" here it would be the diminutive suffix -culus that is attached to minus, the neuter (or adverbial) form of minor.
After a quick search, almost all adjectives¹ on -sculus in Latin seem to be based on comparatives (like longius/longiusculus, plus/plusculus, etc.), so the rule appears to be that -culus is directly attached to the neuter/adverbial form of a comparative. With other words, -culus is normally attached by chopping off a final consonant group (if present) from the word and then using a a vowel that is in the last syllable, or -i-. The combination verna + -culus becomes vernaculus, ferox + -culus becomes feroculus, annus + -culus becomes anniculus.
The word minuscule is often misspelled as miniscule, on analogy from the derived prefix mini-, meaning small. But mini- comes from miniature, which comes from Italian miniatura, which refers to the art of illuminating a manuscript with colored inks. The derivation comes from Latin miniatus, which is the past participle of miniare, meaning "to color with minium", where minium is the Latin name for red lead, a pigment used for illuminating manuscripts.
There is no etymological relationship of the word minuscule and miniature.

¹) All Latin adjectives on -sculus except one are based on neuter comparatives, in Cerberus' dictionary:

altiusculus
ampliusculus
duriusculus
frigidiusculus
grandiusculus
lautiusculus
liquidiusculus
longiusculus
maiusculus
meliusculus
minusculus
nitidiusculus
plusculus
putidiusculus
salsiusculus
suffusculus (the only exception: here -c- is part of the original adjectival stem, so this word has a different suffix, -ulus)
tardiusculus
unctiusculus

