I moved and re-sized four windows in my stucco house this weekend. I can echo what has already been said here. There are tables online to help you size the appropriate header for the window width depending on the window placement, i.e. gabled end, first floor, etc.
Frame the inside first. If you're installing a large window, say wider than 48 inches (that's my gut feel, I'm not sure what the minimum really is), into a load bearing wall you'll want to install a temporary wall to support the load while you remove the existing framing. A temporary wall is a "real" wall, fully framed and nailed with studs 16" o.c. about two feet inside of the exterior wall.
Once the framing is in place I used an angle grinder with a diamond blade to cut out the opening.
The windows I replaced were not flashed, and you may be able to tell from my pictures I really have nothing to flash to/against because the stucco is the only layer of building material. I consulted for a while with Home Depot about the flashing issue, and they've assured me the lifetime warranty on my windows will cover me over any leaks for any reason, as well as the repairs to the inside of the house. I took several quotes from several local window companies from names you know; Pella, Anderson, Marvin, and in the end I was sold on HD, and they weren't the cheapest. However they were the only company to offer a lifetime warranty on the windows for any problem. I was skeptical at first but they've really been awesome to work with.
This is a picture of two of the windows framed in.
One has the exterior plywood cut and installed, just waiting for the stucco guys to come out and patch in new stucco.