There are alot of things to be considered. What is your load span? What is you dead load? What is your live load? What is your snow load? What is your wind load?
(2) 2x12s with a 1/2" plwyood space for 2x4 walls
(3) 2x12s with a (2) 1/2" plywood spacers for 2x6 walls
The above sizes should cover most residential scenarios, but I would recommend getting a professional to review it. Undersizing the member will cause sag and eventually effect the window opening with either cracked glazing or inoperable sashes.
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