Info they gave you is the best.
Bare minimum is that the Length x Width = Snake Length, but ideally we should be offering more than the bare minimum. Males CAN live in 20GL dimensions for their entire life and a lot of people do that without hurting the snake, but if you have resources available anything larger than that for males and larger than 30-40G for females will definitely be used by the majority of animals if you provide enrichment such as more than two hides, activities such as climbing, and ground cover such as fake plants to allow them to move from point A to point B while feeling secure and "out of sight". But having your BP in a 120G? It's gonna be fine (and quite possibly happier for it), as long as you give it adequate places to hide throughout and can heat it correctly.