I'm a new owner too, but I think the rule is 10-15% of the snake's weight should be the weight of the mouse/rat.

Having a kitchen scale should be a necessity...I see a lot of them on sale these days.
I'd assume 6 years would have some good weight, but from what I've been reading, use the 10-15% rule & the width of the food should be pretty close (or a little bigger) to the width of the snake.