I like to give each of my snakes a nice bulge in there stomachs. For me, I increased the size of the rodent by the behavior I observed right after each meal. For example, my retic would be looking around flicking his tongue and would be super attentive to what was going on toward the front of the tub. And, I noticed this behavior a few times even thou he just finished a meal. I also noticed that he was very flacid and I could see his spine. That was also a indication to feed more per session.
For my one bp I just took a chance. I had a small f/t rat that was for at the time for my larger normal. She wouldn't eat it and I didn't want to waist it. So, I gave it to the smaller normal that was normally eating ween rats. To my supprise she had no problem eating it. Now, I give her small rats and she has grown quite a bit in a short time.
I would say just try a size larger and see what happens. Use good judgement and compare your bps widest part of it's body to the width of the rodent. If there isn't a great difference then you should fine.