I have a 2" line stubbed for a drain in an area of a 15' basement that will be framed 8' high with the rest used for storage above. Are these walls now considered load bearing? What's the maximum size pipe I can run through a load bearing 2x4 wall assuming I'm using a protective plate.

I wouldn't think these can be called "load bearing" but I'm not a architect.
Most load bearing walls I have seen are at least 2 X 6 or concrete block.
A 2 X 4 wall measures 3 1/2" I have snuck a 3" stack, (3 1/4" OD.) through non load bearing walls on occasion but it's a tight fit and ya better be "bang on". I'm going to kick this over to experts who are more knowledgeable about building construction then I am. Regards, Tom

Any stud in an exterior wall or bearing partition may be cut or notched to a depth not exceeding 25% of its width. Studs in nonbearing partitions may be notched to a depth not to exceed 40% of a single stud width. Any stud may be bored or drilled, provided that the diameter of the resulting hole is no greater than 40% of the stud width, the edge of the hole is no closer than 5/8" to the edge of that stud, and the hole is not located in the same section as a cut or a notch.
Exceptions: A stud may be bored to a diameter not exceeding 60% of its width, provided that such studs located in exterior walls or bearing partitions are doubled and that not more than 2 successive studs are bored. R602.6(2)