Bigger the better, I have four 1" open holes on my UDS, and use fridge magnets to close them down when needed. You can always close off the intakes, but if they are too small, you can't do much to hit higher temps when you need to.
To be crystal clear.. pipe nipples aren't needed to make a functional UDS. They are a luxury.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)