When to upsize my tank?

I brought home a young bp the other day, she is only about 5 months old and still quite small. Right now I'm keeping her in a 15 gallon tank, but I was wondering when I should get her a bigger tank and what is the best size?

A 15 gallon tank should be ok for a good while. Also for a BP a longer tank is better than a taller tank.

Never knew they made a 15 gal. :weirdface