This is the measurement you need to take if you want your roman blind to fit inside the window recess.  

Use a metal tape measure, cloth ones can stretch and give inaccurate measurements.

Tips

Measure in 3 places for both the width and drop, take the smallest measurement for width and drop.  

Width:  Please deduct 1cm from the smallest width measurement for clearance.

Drop:   Do not deduct anything from this measurement. (Unless of course you require it shorter for any reason)