Installing prehung French Doors in a mismatched sized space

Frame out to almost exact size with 2x4's.

Extend the casing with some nice 1" pine to fit to the outside. If you set the door to one wall, then you will have to extend only on one side. Check the thickness of the casing first, as this may influence how you do the 2x4s

Hang the door, shimming as necessary for a good fit, etc. You can get small bundles of shingles at the Borg for this.

Patch the dry wall. Also, depending on what moulding is on the door, this might cover up most of the dry wall patch work.

You have to frame in the sides to get the rough opening you need. Filling half the distance on a side is generally better appearing than filling only one side unless it is made to be a "feature". Sounds like all you need is a couple of 2 x 4 or 2 x 6 studs, some more sheetrock for the inside and a cover for the outside to match or nicely trim what is there.