The general rule I follow is that I want to feed mice/rats large enough that I see a lump after the snake swallows. But, you don't want to go too large that it may regurgitate.

I wouldn't recommend handling the snake after it eats. I usually wait a day or two so I don't stress them out and possible cause regurgitation.

I have had some of my snakes eat more than one...they were hungry. Feeding them larger rats also allow me to wait two weeks between feedings.