When purchasing plantation shutters, one decision you will be confronted with is what size and style of louver to order.
Most manufacturers of plantation shutters are fairly standard with the sizes of louvers produced.  Measuring from the leading edge to the trailing edge of the louver, the sizes are 2.5, 3.5, and 4.5 inches.  Respectively, they are spaced 2, 3, and 4 inches apart (ie. on center).

compare plantation shutter louver size

The other louver variable is whether you want a flat or elliptical louvers.  These terms refer to the profile of the upper and lower surfaces of the louver.  From a functionality standpoint, both profiles function exactly the same concerning light control and privacy.

Flat vs Elliptical Shutter Louver Profiles

So, let's consider louver size first.  Since larger louvers are spaced further apart inside the shutter panel, less louvers are required for a given panel size.  Less louvers that are spaced further apart results in more sunlight and less view obstruction.  Larger louvers lend themselves to a more balanced look on large windows.  Additionally, larger louvers require more window casing depth to accomodate the portion of the louvers that protrude from the back of the panel when the louvers are in the open position.  A general rule of thumb is that from one size to the next lower size there are about 33% less louvers.
Another consideration is the look that you are trying to achieve.  If, for instance you are trying to achieve a more traditional colonial look, then the smaller 2.5 in louvers fit the bill.  The larger louvers are more of a contemporary look.
Since both flat and symmetrical louver profiles function the same,  the only difference is looks.  This is most certainly a point of personal preference.  Some people say that the symmetrical louvers diffuse the light more softly.  I must admit, however, when standing 10 paces away,  I really could not tell the difference between the two.