To measure the width of your door measure brick to brick then knock off between 5mm and 10mm from your measurement.
In regards to the height of the door, measure from the stone work it is sat on to the brickwork/lintel at the top. Then knock off 5mm to 10mm from your height.
This is the way that the majority of modern door openings are measured.
If you are unsure what type of threshold or sill you will need, you can send us photos of your door wide open from the outside and the inside and we will be able to advise you further.
Please send any photos to firstname.lastname@example.org