In this case, the phrase micro does mean the metric prefix 10^-6, but just "smaller than mRNA". The important unit in biology is nucleotides [1], which are the elements specifying the composition of proteins (loosely speaking, the "atoms of biology"). mRNA typically has on the order of thousands of nucleotides, whereas miRNAs have about 20 nucleotides. The physical size of miRNAs is actually ~3nm. [1]

[1] https://en.wikipedia.org/wiki/Nucleotide [2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3500609/

Typically I see “u” used in instances where it is the actual SI Unit prefix for 10^-6, and leaving it as “mirco” is used when in its more common form for “very small”

was thinking the same. it could be mi instead of u because the other one are not size related

Good one, but not really applicable here. Everyone in the greater domains of biology, biochemistry, bioengineering, and medicine (and maybe more) should know all of these types of RNA