I find that I never use it as it is just easier to let the pit cool off and then scrape it out with a 6" putty knife. Maybe I just don't load mine up with enough meat per cook but drippings have never been a problem for me. Anyway, my suggestion is to put in a threaded pipe and just screw a cap on it till you see if you need it and will use it. This saves you from buying a 2" solid brass ball valve unless you need it. Personally, I would just eliminate it all-together for my next build.
I cooked (15) tri-tips and (16) racks of ribs in my gravity feed smoker for my son's high school graduation party here at my house last year and drained a full coffee can of grease out of my gravity feed smoker! I was surprised what came out. I do not wash that one down with H2o, but just give it a good scrub down while it's still hot. Then scrape out what falls to teh bottom and drain last. I used a 2" ball valve in mine and have had no problem with clogging. But on smaller cooks which is the norm, I only drain off about a cup or less of grease. I tried spraying a bit of water in it the first time that I used it and I got white spots on the door and walls, which went away after the next few cooks and once it re-seasoned itself.
My center feed RF stick burner, I do spray down liberally while it's still warm (not hot) and let the grease and water run down into a bucket hanging off of its gate valve. I like to steam that one out. It comes out really nice when it cools down.
So that said, drain size, or no drain depends on how you plan on cleaning the thing out. You could try DT's method and not even add one and save some $$, and if you hate it later, you could always add one later and figure out what size you need based on the pool of grease that accumulates, based on cook volume, if any.