Mei Long Village's "pork pump"
You don't have to preorder, but it definitely helps as sometimes they run out.
If you ONLY ordered the pork pump, it will probably easily feed 3 to 4 adults (that's including the gratis rice that you can wash it down with). But no one really orders just pork pump, that would be like making a meal out of just guanicale, or drinking nothing but port. Just too overwhelmingly rich.
Lets say you go with a group of 4 or 6 people (or maybe 8 to fill out a nice round table with a lazy susan), most groups of that size that get the pork pump would also get other things such as the "nian gao", or maybe the Lion's Head meatballs, an order of XLB perhaps, and then maybe a vegetable dish like pea shoots or string beans.
The pork pump is big. Easily feeds four as a main entree between its size and its fatty richness. I could be wrong about this, but I think it's the entire picnic ham (i.e. the hog's upper arm). If I had to guesstimate, I'd say it starts off as a good 5 pound chunk of meat, fat, skin and bone.
I've never pre-ordered, it seems they have them braising and waiting for you.