JUST SOME BASICS

I WAS WONDERING IF ANYONE CAN TELL ME THE STOCK SPEAKER SIZE FOR THE SPEAKERS IN A 91 4-RUNNER AND IF I DID CHANGE THE SPEAKERS -- UP TO WHAT SIZES CAN I FIT IN THE TRUCK--ALSO I GET A POPPING NOISE AROUND TIGHT TURNS-I HEAR THIS IS COMMON--AND FOR THE LAST QUESTION--I'M LOOKING FOR A BRUSH GUARD FOR MY FRONT END--CAN YOU TELL ME WHAT BRAND IS A GOOD BUY AND IF IT IS STURDY