Choosing a measurement style is one of the most difficult decisions in the purchase of vinyl exterior shutters. The first step to replacing your vinyl shutters is with a walk or drive around your neighborhood. The first thing to look for is how the exterior shutter tops are placed in relation to the top of the window.
If the area you live in typically has lap siding (wood or vinyl) and wood or metal trim (e.g. a 1" x 4" or a 2" x 4") around the window, are the exterior window shutters flush with the top of the trim or are they aligned with the top of the actual window? If your area has a lot of stucco exteriors and no trim, look to see if the exterior vinyl shutters are mounted flush with the top of the window. In an area with a lot of brick houses, are the exterior shutters mounted even with the top of the opening or are they lined up with the top of the actual window itself? A lot of old homes (now referred to as heritage homes) have a 2" brick-mold that is fairly flush with the siding, how are the vinyl exterior shutters mounted? Take note of how the exterior shutters are typically mounted with respect to the trim and window in your neighborhood.
So now we have covered most of the representative types of window trim, the next step is looking at the bottom of the exterior shutters. Check to see whether your neighbors have their exterior shutters installed all the way to the bottom of the opening or if the exterior vinyl shutters are lined up with the window. If there is a sill at the bottom of the window, it is typically best to leave a 1/4" to 1/2" space between the bottom of the window shutter and the window sill. This gap is primarily to allow for the expansion and contraction of the vinyl material.
While most details become evident in your reconnaissance of the neighborhood, there are some finer points to note. A well fitted pair of exterior shutters that look "just right" may well be custom made for that house.
A quick aside about the value of paying for custom made shutters vs. buying the relatively inexpensive standard sized shutters. When referring to vinyl shutters, the term "custom shutters" means that the height will be a custom size and the widths are only in specific sizes (e.g. 5 3/4", 8", 9 1/4", 11", 12", 14 1/4", 16 1/2", 18", etc.). Standard sized shutters are only generally made in 2 widths (about 12" and 15") and only come in heights that are about every 3 or 4 inches from about 30" to 80". You must consider whether standard size shutters are going to work properly for you, as poorly sized shutters not only look a little shabby they can actually affect the curb appeal (and resale value) of your home. Custom made vinyl exterior shutters may cost twice as much as standard shutters but the return is often worth the investment, as exterior vinyl shutters are one of the lowest cost improvements you can make to your house. Properly fitted exterior shutters can make a big difference for a little extra money.
To determine the proper width of your vinyl exterior shutters there are many things to consider. It is generally accepted in the trade that the style of home influences the decision about the width of the shutter. For a two story home that is narrow to the street that has vertical operating windows you would want to emphasize the height of the structure. You would do this by choosing a narrower width of shutters. On the other hand, a ranch style home that runs parallel to the street would look better with wide vinyl exterior shutters to emphasize the width of the house.
You may start by dividing the width of the window by 2 this is the maximum exterior shutter width you would want for any window. The minimum would be around 25% of the window's width. When you also consider the style of home, you will then have a starting point to work from. In general it works best to find a single width that works for all of your windows, but you also need to consider the space between the windows.
That said there are many exceptions to this rule. For example, if you have an 8 foot wide window on the front of your house with no (or very few) windows facing the same direction, you may wish to order wide exterior vinyl shutters for the wide window, and then order narrower shutters for the other windows on the house. Another thing to consider is if you have a light, down spout, or other obstruction that is too close to the window for a wide shutter to fit. This is where ordering custom shutters can come in handy so you can get a width that works within the distance from the obstruction. Custom exterior shutters can solve many problems that "only seem to be on my house".
A good way to tell how a particular width of vinyl exterior shutters will look without investing a lot of money is to find some corrugated cardboard boxes that are fairly sizable and cut them to the proper height and 50% of the width of the windows. Tape them temporarily to the side of your home on each side of your windows using duct tape, and then step back away from the house to see how it looks. Most people will probably think they look too wide at that point. Take the cardboard cutouts down, and with a utility knife, cut them down to where you think they should be and step back again for another look. You should be able to tell if you went too far almost immediately, and you may also notice that some windows still look a little wide, while others look a little narrow. Take note of this and then take the cardboard down to measure the width. Now you should have a pretty good idea about the width of exterior shutters you need to order.