I would like to cut out a large part of an interior laod-bearing wall that currently separates the kitchen from the dining/living areas. The section of wall I would like to renovate is about seven feet long, and I would expect to frame it with a stud and double jack studs at each side of this opening. Te kitchen currently has a 1-ft soffit above the existing cabinets, and the header would be on the outside of that soffit, so I can accommodate 2x12 dimension lumber. What size/material header do I need?

I would usually use doubled up 2 x 12's glued, bolted and predrilled so the head of the bolt and nut are recessed in the wood. They should also sell premade trusses for this use.Be sure to support the ceiling during demo and locate placment of metal columns before beginning (could be in your opening).

I had a six foot span and sandwiched 3/4" plywood with 2x12's - glued and screwed.

http://i22.photobucket.com/albums/b3...0173header.jpg