I suggest feeding a rat over mouse as soon as you can. My boas do much better on rats. The rats have more nutrition than a mouse 8)
As for size, it should make a small but noticable lump in the snake. However it should not be so big as to stop the snake from moving or curling up. Remember, Royals like to hide so hopefully after eating your royal can still fit in the hiding spot
The way I always read it was that most snakes can reliably handle meals up to 1.5 times their girth, but a meal of equivalent girth, or a little bit under, is an adequate meal and is less likely to cause problems (regurges, etc).