Adult can mean young adult or retired breeder. Which varies in size.

An adult mouse can be around 40 grams.

A small rat (depending on where you get it) can be around 35 to 80 grams. In petstores you often get that on the lower end, with frozen ones usually around 60 grams.

There are simply pretty wide variances. Best bet is to go by the snakes size and aim for leaving a small bump. Usually meaning the rodents girth should meet the snakes girth, both at their widest size.