What size header is needed for a 16 foot opening in an interior wall with trusses above. Wall currently separates kitchen and living room and I want to remove it. Living room is cathedral ceiling and kitchen is flat.

You have not given us much to go on. Is the interior wall running with the headers or at a right angle, what load is on this wall?

Interior wall is at right angle to trusses at the intersection of the cathedral ceiling and the flat ceiling. Standard 4/12 roof load above

This doesn't tell me there is any load on the wall since the truss span is at right angle. The only load/weight I can picture is the weight of the ceiling. Unless I am reading you wrong a double 2x10 should be sufficient. Glue and screw them together with a skin of 3/8" plywood between the two boards.

what side header is required for the following; I want to create a 20' opening on an exterior wall which is built out of 2 x 4 x 8' which support a flat roof. The flat roof is built out of wood 2 x 4 scissors trusses that are 14" in height x 14 feet long. The other end if the wood trusses is supported by and interior wall. The opening I need to create is to be 20' wide by 7' in height. Plan on using an engineered beam or LVL.