You should definitely collect the dust from both the fence and the enclosed cabinet. I just finished my router table with a wye fitting that connects both hose lines to my dust collector. You can check it out in this thread: http://www.routerforums.com/table-mo...completed.html
You will want a 4" port and hose from the cabinet and either a 2-1/2" or 4" port and hose from the fence. A 2-1/2" port for the fence is sufficient, IMO, and is less obtrusive on the top of the table. If you have a manufactured fence with an existing dust port, use the diameter that was provided.
Keep in mind, that when you build your cabinet, you will need air intake holes or a vent to draw the dust out the exhaust port to your dust collector. Also, it is important that the inside of the dust cabinet be as friction-free as possible. I used two coats of paste wax, buffed smooth. Other people have used melamine or smooth laminate for the inside. The less friction, the better the dust collection.
There are a lot of ideas, plans and photos of router table cabinets out there. Google it or check all the woodworking forums (including this one) for ideas on your build.
Good luck. Let us know when you finish it and post some photos!
I'm currently building a router cabinet and wanted to know what do I really need for dust collection. I've never had a cabinet before and wanted to know do I need both suction from under the cabinet and at the fence? What size port is ideal for the cabinet. Should the port go to the back of the cabinet or below the router? My master plan is to add a cyclone in the next year for my garage shop.