This depends on where you want the blinds installed. There are two options:
Option 1: Outside the window frame
Blinds Width = Window Width + 100mm(10cm) this will allow the wall to be covered on both sides.
Blinds Height = Window Height + 100mm(10cm) or (the height you are comfortable so it would be = Window Height + ???mm)
Option 2 : Inside the window frame (the window Depth must be at least 70mm(7cm)
Blinds Width = Window Width minus 5mm(0.5cm)
Blinds Height = Window Height.
Measure the right side, measure the centre and measure the left side.
Specify which side the motor must go for each of the options Left or Right.
If you want to split the lengths then measure the full width of the window first then measure the sizes of the split.