I wouldn't worry about zinc (galvanization) on inlet air pipes unless you're doing something tricky like extending them into the center of your coal/fire source. As long as they end at the drum wall, they're being cooled on the inside and outside by room temp air. I assume you're going to do a "seasoning" run so you can inspect them afterwards and see if there's any whitening of the surface (oxidation of the Zn) at the input to the drum. If so, you might want to change out the final nipple to black iron or brass or SST.
You're usually best off sizing your fittings (meaning your valves?) to be the same size as your piping. (Was that your question?) If you're using two (and a 3rd stubby for fire starting before a proper draft starts) I think you'd be OK with 3/4 pipe, but I suspect most UDS builders would tell you 1". Since this is a bit dependent on the fuel you use and how you stack and space it, and since you say you already have a smoker (UDS??), you probably have an experimental base to guide you on the piping size decision?