If you're trying to compare size with age, you will not get consistent results. All snakes grow at different rates and in different ways. A year old snake could be very short and thick, looking 'older', or very long and thinner. Length and visible size are not good indicators of age.
Heck, even weight can vary within age groups! The only way to really know a snake's age is to know when it hatched.

But, if you're just wanting to see the different sizes of BPs compared to their humans, it's better to see them being held in the hands. Placing any ground dwelling constrictor around your neck is a risk, since they will want to 'hold on tight' to avoid falling.