Toilet vent

Searched but couldn't find my answer. I know(think) a toilet needs a 2" vent but is it acceptable to start the vent out as a 1 1/2" and then increase it to 2"? I have 2x4 framing and didn't want to run the 2" through the wall since it would be a giant hole.

Find out what code you are under. I think it's the IPC in which case a vent specifically for a toilet is not required only that there be a vent somewhere in the system. As to starting out small and enlarging the answer is no

Yea, it's IPC...the link to it is http://ecodes.biz/ecodes_support/free_resources/2012NorthCarolina/Plumbing/12NC_plumbing.html

Maybe that would explain why the toilet didn't have it's own vent already.