Jason, I usually use a three-fly rig and drop each fly off of the bend of the hook using a clinch-knot. As far as size, I don't think there is any hard-and-fast rule as Buddy stated - whatever works in the situation you're in. Usually I will start with a weighted 'Big Ugly' Rubberlegs stonefly imitation as the fly to get the rig down followed by a much smaller bead-head nymph, followed by a fairly small trailer in a smaller mayfly or midge pattern. Catches a lot of fish and works well for me.
There are a lot of other dropper configurations out there and many are very successful, you just need to try them out and find the one that works best for you in your favorite waters.

kelly.