Definitely start small. That change in size of food may puzzle him for a while. Buy the smallest rat you can possibly get, youngest weanling possible. That way the snake will not be intimidated by the first change in prey. Then you won’t have to worry about having to remove the prey item later. Once ingestion of the new prey item seems routine and easy, you can provide a slightly larger prey item for your convenience. Observe the strike and swallow moments, particularly swallow. There should be no struggle, none. If the prey is a little too large, the snake may eventually be successful, mostly. This can cause digestive problems that can have ramifications down the line. Ease into the change of prey item, bigger is not better.