At 8 months mine was still on fuzzy rats iirc, I raised my own so as soon as they got an entire coat of fur they went to him, so they may not have fit the exact sizes that most rodent feeder providers use. He didn't get on small rats until he was around 1.5 years old, roughly.

That said, you want to aim for either no bulge or a bulge with no scale separation in your boa constrictors. Smaller bulges as they get older.