The size listed is the size of pipe that fits in it.
For example I ordered 1 1/2" bulkheads. Standard 1 1/2" pipe fits in side. I had to drill a hole LARGER then 1 1/2 to fit the bulk head in. For Slip fit only. If it says threaded or FPT(female pipe thread) it will fit the threaded coupler of the listed size. i.e. 1 1/2" male pipe thread. Like my male thread to 1" barb fittings.
The O-ring is enough. Just make sure it's snug but don't over tighten it as that may crack it. Its the same type of O-ring set up that shower drains use in Millions of homes everyday, BUT using some silicone once everything is in place isn't a bad idea as silicone is cheap. Leaks can be expensive.