Option 1 – Roman blind fitted on the window frame

Roman blinds have a drop of 27-30 cm when pulled up
The width of the Roman blind equals the width of the windowpane + the width of both glazing beads so that the window handle does not interfere with the functioning of the blind when fitted.
The height of the Roman blind equals the height of the windowpane + the height of both glazing beads.
Option 2 – Roman blind fitted on the window recess
The width of the Roman blind equals the width of the window recess plus 2 x 5 cm (for example) giving you a 5 cm overlap on both sides of the recess.
The height of the Roman blind equals the height of the window recess + 5cm (for example).
The above figures are just an example. You can choose any overlap measurement for width and height, provided the measurements are the same on both sides so that the Roman blind sits symmetrically on the window recess.