Impossible to say without knowing what's on top of the header. In residential construction, a doubled (side by side) 2x8 is legal when there is just a truss roof over it. I'd go double 2x10 if I had room just because I'm one to put in all of the wood I can get in there. If this is an older home without factory built trusses, or has an upstairs, a floor in the attic space, used to be an exterior wall, you better go talk to a pro who can look at the house and actually see what you have going on there.
Welcome to the forum Texastom!