run it through a capillary sequencer with genotyping size standards. The standards are not accurate at low molecular weights but will allow sizing analysis and if you make an oligo ( fluorescently labelled different colour from your sample ) you will get an accurate idea of the size of your rna if it is less than the lowest size standard in the commercial sets. Thhe rna will need to be desalted to make sure it loads correcly on the capillary