Hello Tara welcome to the forum!
I have done a few RNA emsa's... are you testing for RNA splicing or UTR untranslated region (5' or 3' UTRs ?) - translation interactions ?
your cDNA may or may not have the RNA spicing or UTR areas ... please mention which area you will study....
300 bp is fine for now, you must clone it into a T7 / T3 construct and express it as an RNA... you must then take cytosol and add it to the RNA in proper buffer (salt) conditions.
Eventually you will need to make smaller probes to find a more specific area of interaction. You might also try non-radioactive EMSA. Pierce makes a good Light Shift Chemiluminescent kit which can be used for RNA EMSA as well as DNA.
please respond for those questions and we can help more!