Let me start off by introducing myself. I'm SOS and have a longstanding history using and abusing gear. I do a lot of writing and reviewing for us in house and you may have seen some of my posts on the forums. I also put everything up on our Facebook and Twitter accounts that you like. The stuff you don't like is obviously some other guy. I also like scotch and ruining my own life, now, back on track.
With introductions out of the way, let's talk about plate carrier sizes and fits.
One question I get over and over again is, "I'm ready to buy my plate carrier, but I don't know what size to get." Some people don't realize that there are a lot of factors involved. First off, plate carriers (otherwise referred to as PC, not to be confused with Personal Computer - those are only used for porn and ordering sweet gear from us) should be ordered in the same size as your plates. If you have medium plates, then the correct size PC would be a medium. I know this sounds like a real "no shit, Sherlock" moment, but trust me it happens. Our PIG PC is slightly different as it can accept a wide variety of plate sizes due to certain design features.
The main problem a lot of people face is taking proper measurements, or even understanding what to measure. Based on everything I've seen, this happens a lot and is a source of confusion and worry. The knee-jerk reaction that ends up occurring are people ordering a larger size PC than needed based on the incorrect belief that you'll get more coverage and more protection.
I've found that sizing up on PC fit ends up leading to two serious issues;
1. The first issue being that you can no longer get into your proper shooting stance, and can therefore no longer properly employ your weapon. Unless you don't mind missing tangos, you can see how this would pose an issue.
2. Secondly, is the loss of mobility and the added weight of the extra plating and material. This can have significant impact over the long haul. You aren't just wearing plates - you are fighting in them.
You don't want to be that armored turtle, heavy on defense but not capable of offense. Finding that middle ground of protection is where you want to be. My general rule of thumb for minimum coverage is from my jugular notch to my belly button vertically and nipple to nipple horizontally.
This is what your plate carrier should be protecting:
This is the minimum amount of coverage and allows you to sit comfortably, properly punch your weapon out, and be able to move with the least amount of hindering. You can go larger but always remember the balancing act of offense versus defense and that it is all a give-and-take scenario.
Once you have measured that area on your body, make cardboard cutouts of different plate sizes, and try them on and see what works best for your body type. Going through these extra steps add some time to the process and may feel a little silly, but I highly recommend taking the time. Once you are comfortable, you can confidently order your proper plate and PC with confidence.