Ok, that's understandable.
Firstly, make sure that your aperture is not crooked (careful, Fnarr!).
To do this, hang a plumb down each of the verticals on both the inner and outer skin.
Then chop back any exterior render to the brickwork (use a brick bolster and a hammer). You don't need to chop internal plaster back.
Then take 12 (yes, 12!) measurements, as follows:
Internal skin:
Horizontal at the top, middle and bottom of the aperture (3 measurements).
Vertical at the left, centre and right of the aperture (3 measurements).
Repeat the exercise for the external skin.
Remember to measure brick-to-brick (not the size of the current frame).
Also remember, "measure twice, cut once!". So repeat the whole process (24 measurements!) to make sure you're double-sure.
The size of your external aperture is the smallest width measured x the smallest height measured.
Repeat for the internal aperture.
You may have a larger internal aperture than external (my house certainly does).
The size of frame that you need is as follows:
Height = height of largest aperture - thickness of sill - 5mm.
Width = width of largest aperture - 5mm.
You will extract the old frame and insert the new one via that aperture.