Scarus quoyi has the potential to exceed 15" long. They are heavy bodied, active fish, and would require a large system with a Very good filter in place.
They are one of the few parrotfish that do not rely on hard corals for their diet- they actually eat plants/algae. However, they are very likely to chomp down on stony coral in a reef tank, just to get to algae growing around the polyps if nothing else. It can devastate a reef tank even though it does not eat coral as other Parrot Fish do. That said, Quoy are probably one of the best species to try if you absolutely MUST have a parrotfish. Best in large fish-only systems.