If you just give it time, your snake will get big, trust me. I cheated with Goblin, by buying him when he was already adult sized, but my other 2 Bps are only about two and three feet long, and were tiny when I got them. Overfeeding is not a reliable way to grow a snake fast; it usually just leads to regurgitation, and that will just hinder growth. Just keep to your regular feeding schedule (I think you said, every 6 days, that is good) Switching to rats definately helps too!

Goblin is the one who's really big; Gunth is still growing; he's maybe 3 feet. But Gobby is over 4, and he is really wide! Here's a pic.