Hi,

Where did you get the counts from. If it was a gtf/gff or bed file, you can use it to extract the length. Otherwise, you can use for instance biomart or RFAM to get the information you need.

Nevertheless, you should think of using a different normalisation method, since miRNAs' length is usually a fraction of a kilo-Base, boosting the RPKM to high values even for a few reads. See for instance here.