At 6 months she can no doubt take a small adult mouse.
Weighing her is handy too.
There are two basic rules of thumb regarding prey size for BPs.:
1) the prey item should weigh 10-15% of the snake's body weight
2) the prey item should be about the same girth as the snake's widest part.
Keep in mind these are general rules of thumb only.
Also, this changes a bit as they reach adulthood.