I don't think many of them post that info on their sites, but if you called some of them (such as SVS), that is typically one of the first things they will ask you and they will help you determine how much sub you need. HSU's recommendations are just that, as different shapes and construction types can affect the sub's performance.
If you have no experience with a sub in your room, then you as the buyer would have a pretty tough time determining it, but the first place to start is usually how many cubic feet your room is.
