David, they are 2-1/2 inches for a regular DC hose. I noticed that the box for the Super DD was larger than what my DD would be.
If you are going to use the HF dust collector vs a shop vac, you would need to upgrade to the Super Dust Deputy. Also, if you go up to the dust collector, if you use a 4" to 2.5" hose adapter you also need to figure out someway to add a 'make-up air' gate to keep the collector and the Super Dust Deputy operating fully. The Super Dust Deputy provides a few choices for input and output size, but all of them are at least 4", allowing the cyclonic action of the unit to function due to the volume of air going through the cyclone.
If you constrict the airflow by using a small adapter (say 2" or 2.5") to make a connection to a tool, the dust collector will be starved for air and the cyclone action will be seriously diminished causing your filter media to clog quickly and your blower to overheat. On mine, I use a 'Y' adapter that I configured with two blast gates on the top of the Y (2 in ports) and the dust collector attached to the bottom of the Y (the outport). The one input blast gate has a 2.5" adapter to 4" for connection to my sanders and the other has no adapter just open to the air. I use the 2.5" blast gate fully open and slowly open the 4" blast gate to provide the make-up air for the blower. You can hear the change in the blower noise when you get it close - open it too much and the 2.5 side loses a lot of suction. Blending it gives you the best of both worlds.
A DC works on air volume, a shop vac works by velocity of the air so you have to always consider the efficiency of the DC to be able to 'breath' to be able to run correctly. Length of piping, smooth pipe vs ridged, and the number of changes on direction all impact a DC, so be sure to consider how you will ulitmately connect your DC to your tools or work area. Check out the articles on the web by Bill Pence for more info on dust collection building and piping.
