Not sure what to say other than our bp was 17" when we got her 2 months ago and was eating adult mice from the get go. Not the bigger retired breeder mice but more the normal young adult mice. She's now about 25" long (sorry no current weight) and is taking 1 or 2 mice per week.
I'm sure a more experienced owner would be better to advise you than me about what is appropriate for a snake of that length and weight.
Ours takes her prey head first but it's hilarious to watch Baby fuss and drag her mouse around by the tail if it's not in the exact postion she wants it in LOL.
~~Jo~~