Originally Posted by benjeeman
Rule of thumb is that you can feed an item that weighs 10-15% of the snake's weight.
Or you can look at the broadest part of the snake and feed an item which is the same size or just slightly larger.
And as for when to switch from mice to rats most people would advise as soon as possible, it prevents the snake from getting too used to mice and potentially being finnicky with the switch.
In addition, rats are considered to be more nutritious and will therefore aid healthy development/growth of the snake.
Hope that helps.
Whilst a lot of that is true, the skeletal structure of mice for a given size will be more developed when compared to a rat at the same physical size, so consider sticking with mice for now.
However there is also no real need to migrate from Mice to Rats. African Soft Furred mice (often called multis) are the snakes natural diet, and these are high in protein and less fatty when compared to traditional lab mice. An Adult royal will still be content on one large ASF Mouse a week. - something worth considering