If he is young know that the ideal prey his a prey that his the size of his girth, but ideally until you get a hang of it and know what the prey should look like get a weight and follow this.
https://ball-pythons.net/forums/show...=1#post2493722
As some point after you have fed him over and over again you will just look at a feeder and know if it's the right size, if you are unsure weigh the prey and weight the snake, because remember young BP are programmed to eat and you want to feed them the right amount so they can have a normal growth.