With the wearing of hard armor to defeat rifle threats becoming both more common, and more affordable, there seems to be conflicting opinions on plate sizing. One school of thought argues for maximum coverage- setting up the plate carrier with oversize side plates, and 11X14 (or even larger!) front and rear plates. The argument being, the greater area of coverage will result in greater survivability.
The other school of thought stipulates that the more steel or ceramic you strap on, the less mobile you will be. Smaller (8X10) primary plates, no side plates, or even omitting the rear plate, are all suggested to lighten the load, or to allow more ammo/sustainment gear to be carried.
Both schools have their merits. However, the latter school has a slight edge in my opinion (your mileage may of course vary). Smaller plates, while not providing as much coverage as larger plates, still do a good job of covering “the box” (Cardio-pulmonary box, containing the heart, large vessels/arteries, and a majority of the lungs). The role of armor is to allow you to stay in the fight longer, not make you invulnerable. A lighter, smaller plate improves mobility, resulting in less fatigue and more combat effectiveness. Not getting hit is always preferable to standing and taking rounds.
Secondly, omitting the rear and side plates (unless in a situation requiring the wearer to be stationary/defensive), may encourage a more pro-active/agressive mindset. Keeping “front towards threat” is not a bad habit to cultivate.
So unless you envision yourself in a fixed defensive situation, it may be worthwhile to consider lightening the load, and choosing smaller/fewer plates.