My remodel bathroom question is concerning combining vents in the attic for one roof penetration point:

The toilet and vanity sink have one 2" vent; the shower/tub have one 2" vent.

I would like to tie them together in the attic-

Do you have to increase the size of the pipe vent (like you do with drain lines) to a 3" where they tie together or can the vent be a 2" all the way (even though you have combined vent lines)?? I can't find information on this anywhere. Thanks.