you can span more, but you need to sister several additional joists to your end joists to treat them more like a beam. Then the opening can be framed out like a built up beam tieing into the new built up joists. Roof loads, snow loads (if any) are other things not taken into account here.

I think I get what you are saying, and I plan to do that. I figure, if I remove 4 joists, I need to add 4 joists sistered at the edges of the opening (two on each side).

I am not so worried about supporting the plaster and live loads, but curious about top plate spread/rafter sag. Can you mitigate this with strapping on a diagonal from center of shortened joists at the top plate to the new doubled/triple sistered joists? Maybe this is just crazy talk from an engineer (non-structural). But if I interrupt the joists in tension over a 6 foot opening (breaking the rafter truss triangle, wonâÂÂt that 6 ft opening begin to spread regardless of how many joists I put on either side? Is everything is stiff enough where this is not a concern?

not sure I understand your question or concern exactly. Most loading on the rafter is compression rather than tension. You have tension when the wind is blowing hard from a specific direction on one side, while the other side is in compression. The roof sheathing creates the diaphram and locks it all together. There really isnt a whole lot unique about what you are wanting to do or trying to do. Its a typical roof framing opening for dormers, just no dormer.

You can and you must support the now cut ends of the rafters.

Here is another thread discussing something similar.

http://ths.gardenweb.com/forums/load/remodel/msg031627448565.html

Here is a link that might be useful: GardenWeb