To support an attic load over a 9 foot span, a couple of 2x10's "should" be sufficient. However, you need to figure out how you are going to support these 2x10's. The load they carry is going to be transfered to whatever they are sitting on, in this case I would vote for about 3 2x4's sistered together at each end, but I have to ask you what they are sitting on? If you have a concrete floor, you "should" be fine. You can't just let the load transfer to a 3/4" plywood subfloor or you are likely to risk punching through the floor and down to whatever is below that.
To accomplish your project, you will need to install "cribbing" on either side of your project to support your rafters while you take out your wall and install your header. Your cribbing can consist of sistered 2x8's supported by posts (sistered 2x4's at least 1 pair at each end, maybe 1 more in the middle.) which will sit on a 1' chunk of 2x8 or 2x10 to spread the load on your subfloor below.
I use a hydraulic floor jack and a 4x4 post to lift the temporary headers enough to fit the posts underneath, but understand you don't want to really "move" the header up, just a little, like 1/8th of an inch, then let it back down.
Because you are dealing with load bearing walls, it would really be best to get a permit for your project, and have your plans reviewed by a professional before you get to far into this. Your homeowner's insurance policy won't necessarily cover "acts of stupidity".
Lastly, be aware you could run into power, water or waste lines in this wall. when you remove the wall, start out by removing wallboard from one side, then cut away your studs AFTER you can see what is inside the wall. You may not be able to reroute a waste line, depending on your situation, so don't over-commit yourself until you know for sure what you are dealing with.
Good luck, and please post again if you have more questions...