I am trying to design PCR primers for miRNA but I am having issues getting the right primer length as well as the optimum amplicon length. I need help with blasting the primers
Because miRNAs are so small (19-22nt), it is not possible to design primers for them using conventional primer design software. You need to use a special reverse transcription strategy to make cDNAs which are long enough to meet minimum amplicon size requirements. The two options are ligation of linkers to the 3' ends of small RNAs, or reverse transcription using stem-loop primers. If you google 'miRNA qPCR' you will find several papers giving examples.
I assume you're trying to do qPCR but maybe I'm wrong - what is the purpose of your experiment?