It is quite tricky to size-select the small RNAs from the total cDNA because they are too close to the primer-dimers peak (looking on the BioAnalyzer plot). You may try to enrich for them by doing the double-sided beads - incubate the library with the 0.5-0.6x beads first to bind the biggest particles, and then transfer the clear supernatant to the new well with 0.5-0.7x beads to catch everything else which was not bound on the first step.
Anyway, the best way to separate them might be to use the systems like PippinHT, which allow to separate very narrow fractions.