I know there are many different opinions on whether mice are better then rats or vice-versa. There are pros and cons to feeding each.
What I have done is feed a combination of both to all snakes that weigh in aproximently 40g and up. They may get two rat feedings in a row, or several mice, then a rat, and back to mice again. It varies month to month depending on what I produce from our mouse and rat colonies and what I have available frozen.
Some snakes will not take rats at all, others will not want to switch back to mice once offered rats. It depends on how picky the snake is, and how often you are offering one or the other.
As far as size goes, regardless of the stage or type of prey you are offering, the size of the offered food item should be aproximately 1 and 1/2 the width of the snake. (roughly the stomach area of the snake or middle of the snake - NOT the neck area directly behind the head.) When your snake has eaten, there should be a visible bulge around the stomach area once the food has been swallowed. If you can't visually see where the mouse is sitting inside the snake, you are feeding meals that are way too small and it's past time to bump them up in size.
Many breeders recommend certain stages of mice or rats depending on the age/size of the snake because that is what, with many years of experience, thier snakes ate those sizes at. Each snake is different though, some grow considerably faster and bigger then other snakes. So, while I wouldn't discount the recommended sizes offered by breeders, I'd also take into consideration the snake that is being fed and feed either slightly bigger or slighty smaller depending on the size of the snake.
An example I can give from personal experience is this:
I have a yearling sunkissed male that is eating juvenile adults or nearly full grown adults - he's been eating at this size since at least november/september 2006.
On the other end of the scale, I have a 2 yr old that is still on small hoppers/rat pinks/pups and he/she was just bumped up within the last 3-4 months. This one started out as a non-feeder though, and has been an extremely slow grower all of it's life.
The rest of our snakes are somewhere in between the two depending on age and size.
The next thing that should be considered, especially when chaing the diet of your snake, is the availabilty of the new prey item. If you do not have a reliable and steady supply of the new prey item it might not be worth it to switch as there are times a snake will refuse to switch back to the original prey.
In the end, it is up to you to decide which way you prefer to go. You know your snake best. Best of luck!
Jenn