Just use resolving gel, no need for stacking. You may want to use some kind of denaturing conditions if you care for size and native conditions if you want to see how many conformations your RNA will fold to. You can find basic protocol here:\

http://cshprotocols.cshlp.org/content/2010/6/pdb.prot5444.full

But keep in mind that there is a load of PAGE and agarose electrophoresis protocols, depending on what you want to investigate. This method is used not only for determination of size or number of conformations of the RNA molecule but also to find and quantitate interactions (as gel-shift) with other bio-molecules or with other RNAs, Also for detecting where other molecules binds to RNA (in protection assays).