Not enough information. There is a lot that has to be taken into account in sizing a header of that length, specifically the imposed roof loads. My hunch is that 2x10's will be inadequate. Rafter span and roof dimensions have a lot to do with it. My suggestion would be to look away from 2x10 dimension lumber and give a harder look at LVL or PSL engineered lumber, which will span farther than dimension lumber. Visit a good lumberyard (not a home center, a lumberyard) in your area that sells engineered wood...Most of them have a designer with the capability and software to size the beam for you.
That being said, it is always best to hire a structural engineer when creating such large openings in bearing walls in a home. Interior and exterior bearing walls are often part of the structure's shear resistance, and creating large holes in shear walls is often problematic. An engineer can offer solid advice whereas the guy at the lumberyard can't. A permit is most certainly required by the residential code for this scope of work, and I'd be prepared for the inspector to request an engineer's written recommendations.