Proper size of header to support new door in load bearing wall?
International Residential Code 2012
Chapter 6 Wall Construction
Section R602 Wood Wall Framing
R602.7 Headers. For header spans see Tables R502.5(1) and R502.5(2).


So if you're on the top floor, you can use 2 2x4's (unless the building is 36' wide, in which case you'll need 2 2x6's). If you have a floor above, you'll need 2 2x6's (unless the building is 36' wide, in which case you'll need 2 2x8's).