I use a 2 1/2 inch hose to collect dust while sanding. I am making a kitchen utensil holder with the inside about 6 inches deep and 4 1/2 inches diameter. to sand the inside I was using the usual 1 1/2 inch dowel with a 2 inch slit in the end to hold a piece of sandpaper. Every minute I had to stop and vacuum out the dust so it wouldn't clog the paper. Then I had a flash.
Taking a piece of 1 inch PVC I cut a slit in one end to hold the paper and tapered the other end to fit inside an adapter going to 2 1/2 inches. Now I hook the dust collector hose to it and it keeps the dust out while sanding.
You CAN hold a piece of sandpaper and sand the inside, but after a catch and nearly wringing my arm off I gave that up. And there is still the matter of the dust.