I'm using 2" DWV pipe for bathroom/kitchen sinks and vent. I'm in the
process of constructing the walls. Can I get by with a 2x6" wall or
do I need 2x8"? I will be boring the studs to accommodate the pipe.

As Wayne mentioned, the largest hole you can put in a load-bearing stud is
2.2" (40%), or 3.3" (60%) in a non-load-bearing stud. If you double-up the
studs, you shouldn't have a problem with 2-1/4" holes (41%) for a short
horizontal run. I had to run a 2" drain for our kitchen sink through an
exterior 2x6 load-bearing wall under a window to reach the vent/drain off
to the side (about three feet run total). My inspector said it was fine.

As for using 2" pipe when 1-1/2" would do, I did it for simplicity. It's
easier to pick up a supply of 2" pipe and fittings than to keep an
inventory of various sizes on hand. Especially when you're working a long
distance from the nearest store. I plumbed my entire house with 2" and 3"
drains.

You generally need the equivalent vent as the main drain leaving the
building. For instance, if you have a 3" main drain, you need a 3" vent. Of
course, you could use three 2" vents to get the same venting as a single
3" vent. On the other hand, if you live in a cold climate, smaller vents
can ice over and potentially block the small vent. So 3" vents are often
the minimum size in cold climates.

In my case, I wanted to minimize the number of pipes penetrating the roof,
both for appearance and the increased potential for leaks. So I installed a
single 3" main vent through the roof, and tied all my fixture vents into
the one main vent. I have a large 3" trunk line running through the attic,
and all my smaller 2" vents tie into that. It meant a little extra pipe,
but PVC pipe is fairly inexpensive and the tradeoff was well worth it in my
opinion.

Anthony