Weanling can be a deceptive term as it really depends on just how old the rat was at weaning. Some breeders wean as early as 3 weeks, some as late as 5 weeks so that can mean a huge difference in size. If your snake is used to mice you might want to try a nice rat pup first as a smaller step up. Usually a large adult mouse and a nice sized rat pup are about the same volume of prey (though again pups can vary greatly in size due to genetics, amount they eat, nursing from the female and the actual age....pups are from day 14 or 15 until they are weaned so there's a huge growth spurt in there too).

Basically you want to compare the rat pup/weanling's greatest girth, it's hips and butt area, to your snake's girth and pick a rat that is in size no more than or better yet less than the girth of your snake.