Blind Size or Recess Size, when measuring for made to measure blinds, you need to consider the space available and the position of your window before deciding whether to fit the blind in the recess of the window or on the outside. When measuring always use a metal tape measure and use centimetres (cm) or millimetres (mm).
If measuring for a recess blind, measure the recess width and drop in 3 places and pick the narrowest width and the shortest drop if your window is out of shape. Also, when choosing how far forward in the recess you want the blind to hang, you need to make sure that the blind will clear any handles.
These blinds are fitted to the outside of the window, they will be larger than the window, as the blind will hang from above the window and will be wider and sometimes longer than the window itself, a general rule of thumb would be to measure from 3″ (7.5cm) above the window and the same either side of the window, Although if a window is short and light an issue, plus you have space above the window, the blind can be fitted higher to give an illusion of the longer window.