The Sd card needs to be the same size as your phones internal memory. It would do you good to really read and research before messing with your phone. This particular section (Verizon s3 troubleshooting and questions) is FULL of threads about people bricking because they failed to read that you can't downgrade or you'll brick. I know that doesn't sound very nice, and please don't take it personally...It's not meant to be.
Sent from my SCH-I535 using xda app-developers app