Most windows are a standard size, 2'0, 2'2, 2'4, 2'6, 2'8, 3'0
With most wood window and probably most vinyles, just measure the window pane, in feet and inches, should fall on one of the standard measurements ... 2'0 2'2 2'4 2'6 ....3'0 etc.
If your just replacing the the window and not the window frame, then i would remove the window and measure it.
If your replacing the whole unit, window and the frame, then you should remove the trim on the inside, and measure the outside dimensions of the frame.
Framers use the header size to size the window opening, we take the window size plus 5 or 5 1/2 inches to figure the header size,.. a 3'0 (36 inches) window (or door) would have a header 41 (or 41 1/2) inches, making the rough opening 38 inches, leaving about a 1/4 inch wiggle room on both sides and the top.
williamh
October 2004