I have designed MicroRNA forward and reverse specific primers which I found through other papers. But in order to do PCR i need to know the mirRNA's size. I could't find any size in the papers. Is there any web based tool which I can get the size and could you explain the method for it? Some companies said it could be about or less than 100bp using a molecular marker. But how I know exactly my product length prior to gel running? Any answers are appreciated.