Its difficult to tell you how big to make the pit because we have no idea how many gallons per minute we're dealing with. Essentially you want to size the pit so that the pump will remain off at least 2 minutes before restarting to drain the pit. If your not sure error on the side of caution and make it bigger.
The area you can count as being part of your sump pit is the area below your perforated drain pipe, maybe even below the lowest point of the gravel. Otherwise your perf pipe will fill up with water before the pump would kick on.