What strength header is required at roofline to support roof over 76" and 72" 3-panel replacement windows? Is an engineered beam required? There may be as little as 6" from top of window to roof. The upstairs is finished with full bathroom overhead near where these windows will be placed. This is in Maine with potential snowload. Thanks for any help offered.

With 6" about all you can do is 2X6. Consult the attached table.
Headers & Beams : Size Selection Tables - Southern Pine (http://www.southernpine.com/span-tables/headers-beams-size-selection-tables/)

You stated '' from top of window to roof''. Is this the bottom of the rafter at the seat , under the top wall plate or on top of rafter on the outside where shingles are located.

The required header size in middle NY on a 72" span is 2x10 with double jack studs.

Check out manufactured beam for size. Or you can sandwich3/8" finch plate between the 2x6

Chuck