Seriously, if you're feeding fuzzy mice to a 2 year old snake, then they will NOT be anywhere close to reaching their mature size. And may never reach it if that sort of feeding regime continues.
It's pretty easy to freak out about the correct size of prey item when you look at how tiny their heads are....but they were MADE to eat prey larger than their heads. They can do it, I promise!!
I can't recommend a specific size animal because I have no idea how big your snake actually is....but I can, with confidence, suggest that at the very least, he should be eating large adult mice...maybe two per meal once he gets used to that size prey.
As for the live vs pk debate....feeding live is FINE so long as you do it responsibly. Don't put a hungry, thirsty, freaked out rodent in there....and never leave one alone with your snake for an extended period of time. If they don't eat within the first 20 minutes or so...then remove the rodent.