Measure the width of the opening from edge to edge, horizontally, in three different places: Near the top, near the middle, and near the bottom.
Next, measure the height of the opening on the right side, middle, and left side.
Finally, measure the thickness of your wall, or the depth of your existing jambs. Again, measure near the top, middle, and bottom.
Take the smallest width and height, and the largest thickness. These will be your exterior dimensions and jamb depth. This is the maximum size of unit that will properly fit in your opening.