Like Nicolai said there is no one answer, its going to depend on the amount of initial data and the change rate of that data. Data Domain does a pretty good job of efficiently replicating data over the WAN but if this is a new DD setup for you I would advise placing both Data Domains next to each other for the initial sync and then only worrying about sizing for the change rate. Once you know your daily change rate and backup window (or network window) you can then size what you need.
