A process simulation program cannot size V/L separators - you've got to refer to the plant owner's design guidelines for this.
If the plant owner has no design guidelines, ask a senior process systems design engineer in your company. In this case, you should document these made up guidelines and get the plant owner to accept them before you start work on this.
HYSYS _can_ size your separator - whether you trust that or not is another story. When you have the separator block open, go to leaf "rating" and fill in. So it depends on what you want to use it for. I think the sizing method is pretty standard - same as any process engineer would use in a spread sheet (Navior stokes etc)
