in my opinion if the snake was able to get it down and he keeps the meal down then he's feeding it the correct size because any snake that feels a meal is too big for him if the head of the food cant get in his mouth after 10-15 mins they give up. if it gets half way down but the snake feels like its too big he'll spit it back up and believe me the smell is awful lol...Like i said before if the snake can kill/swallow and keep it down the food size is just fine.
also its a WC as you stated right? so if he got this meal down with no problem and he keeps it down this might be able to tell you what size food the snake was eating before he caught it. id have different thoughts on i if it was CBB.