My ball python is around a year and a half old, and he's 6 inches around (circumference)
I've been feeding him two rat weanlings once a week.

However, I need to upgrade his prey size. I'm not sure if I should go with the small rats or the medium rats.
The small rats are 4.50 - 6.00 inches in length (not including tail length) and 45.00 - 84.99 grams each.
The medium rats are 6.00 - 8.00 inches in length (not including tail length) and 85.00 - 174.99 grams each.

I just recently purchased a gram scale to weigh my snake, but I can't weigh him for the next few days, so I'm not sure how much he weighs in grams.

I'm just looking to see if anyone could give me advice on what rat size would be best for him, thank you!