Hi, we need to replace the backflow preventer its Wilkins 720 A, it broke due to freezing. But I can't really understand when it comes to size, what size are they talking about. Usually available are 1", 1.5" 3/4", 2". Are they talking about the pipe size? How do I find out?

I am first time homeowner, so I've no clue about any of this. Any help would be greatly appreciated.

Thanks