Determining the shutter width and height is very important. Most people Measure from the top of the window frame to the top of the sill, if it has one and they determine the height. Some choose to have the shutters appear as operable and prefer to have the shutters a bit smaller than the complete frame. Whether you go a bit larger or smaller on the shutter, the big thing is to make sure you stay consistent with the rest of the house.
Unless you want your shutters custom built, the Master Shutter series is your best value. These are standard Raised Panel and Louvered Shutters. These shutters are only available in 4 inch increments. This means you will usually have to adjust your shutters a couple of inches one way or the other. Again, just be consistent. Hit it pretty close on window height and you'll be OK.
Standard Shutter Width is 14 to 15". Again the Master Shutter Series is available in these widths. This size will work for most window sizes. However, if your window is very narrow or very wide, there are many other options available. Please refer to our Shutter Width page for details and chart.