If the rat is definitely smaller than the widest part of your snake it will fit. It's one of the most amazing things about snakes in general. With nothing but a mouth backed by a tube of muscles they can swallow things much larger than their head and neck in one piece.
If you aren't sure in the future, you can always weigh the food a few times to help you get used to what he can eat. 10% to 15% is a basic guideline. You do that a few times knowing it'll work, it'll help you start being able to eyeball things more confidently.