Don't know if it's *too big* for an adult Rosy, but most Rosies can be kept in 20 gallon longs (otherwise called 20L or 20 long) for their whole life. Which is smaller than the boamaster you're looking at. Also, Rosies have completely different husbandry requirements than most other boas, you might end up running into humidity (too much of it) and possibly temp (too high) problems with a boamaster. Boamaster and Animal Plastics and Boaphile and etc are more designed to house the larger more tropical boas and pythons.