How to Choose a Proper Size: For INSIDE mount: Choose the size SAME or CLOSEST to your window width. For Example if your windows are 30 wide order 30 or slightly less for internal installation. For OUTSIDE mount: We recommend Shades Width=your window width + (3-5)inch for better coverage. For Example if your windows width is 30 order size 34 would be okay for outside mount. Shades height can adjust from 0 to 48 .