Im planning an an eight-foot framed opening in an interior wall between kitchen and family room. Interior wall runs parallel to the rafters supporting a gable roof. The wall is directly under one of the rafters. Is the wall load bearing? What size does the header need to be?

Load bearing walls run perpendicular to the floor joists and rafters. The wording to part of your question is a little fizzy "paralell to the rafters supporting a gable roof" Rafters run perpendiuclar to the ridge of a gable so I don't know for sure what you were saying but in any event your wall does not sound like a load bearing wall.

even though I would use at least a 2x8 beam made of two pieces of 2x8 with a piece of half inch plywood sandwiched between them for my header.