I feed rats that are bigger than the width of the snake, probably about up to 50% bigger. They can handle it, it's amazing what they can eat. You should look up some videos on YouTube and you'll be shocked at what they can pound down.
How do you thaw your frozen thawed? The temp needs to be perfect. I thaw overnight in a fridge and then put in an incubator at 110F for an hour or so, works great. Just don't get it so hot that you cook the rodent LOL. I like fresh killed best because if they don't eat it you can freeze it for later. When I thaw my fresh killed within a couple weeks you can't even tell that it was ever frozen.