I live is a colonial 2 story home. I am currently remodeling my kitchen/dining room. I currently have a header of 68 inches on the load bearing wall between my dining room and living room. I want to open that up to 84 inches. Can I use dimensional lumber to frame out the header and if so what size? Or do I need to go to a Lam Beam?

Are you planning to pull a permit for this? You'll need a structural engineer to size the header, especially if it is a load bearing wall.

If it wasn't a load bearing wall you don't need a structural engineer at least not here. There are standard accept building practices and basic load capacities for dimensional lumber

Your only going 16" more. Most Headers we use over windows are the size they are simply because if that window of 30" could take a doubled up 2x6 to span it we still use a 2x10 simply because it easier to do instead of cutting mini studs over that header.

If you go to the micro lamb side here they can tell you the load capacity on span versus height. Or you can mini micro lamb a beam from the dimensional size you have there now and sandwich ply between or or buy a steel I beam.

An engineered may cost as much as $2,000.00 to say what yup its OK what you want to do.

The big question here is where the weight transfer is going moving thoses Jack studs and what's below level for support and may need house jacks in basement moved or added...