I am working on an attic buildout now on a house roughly the same age. It is a shotgun (straight thru Bath in back). 2 x 6 @ 16' on both sides of house resting on a six foot wide hall right down the middle. The walls of which being structural.
The attic area is approx 968 sf (Large). We are putting in 1 additional dormer, master bdroom, walk in closet, skylights (4) and a master bath - as well as stairway. The ENGINEER (always consult on such a project) recommended 2x6 sistered with 2 x 10. (BTW they are 16" on center as they should be).
The ceiling joists under the upstairs bathroom must also be supported more, by header beams and to make this look good to the homeowner, it will be finished with nice select pine.
In addition , the rafters must also be sistered with 2x6 ( they are 2x4 now.) A good idea for several reasons, mainly the insulation and channel vents (the roof is a "hip" roof). Also an additional support is required left center for additional roof support when they are removed after building the knee wall.
Not to be long winded, but there are lots of reasons to consult a structural engineer. They will come to your location. The fee would probably be in the neighborhood of $200 - at least that is what it is in Atlanta. If your going to do it - do it right. Lots of furniture= lots of weight and traffic. It would be money well spent.