Your plate carrier should ideally be snug, but not so tight that it restricts breathing or movement. Keep in mind that when you are in a self-defense situation, your breathing will be heavy, and your chest will expand a lot more than when you are breathing normally. So, if your plate carrier is too tight, it can cause discomfort when your breathing rate increases.
At the same time, your plate carrier should not be so loose that it shifts and moves around when you are running or doing any other activity.
Ideally, you should wear your carrier, and take some deep breaths. Your plate carrier should just start to feel tight when your chest is fully expanded. An excellent way to determine the fit of the plate carrier is to simply do a few burpees and pushups while wearing it. This also incorporates a wide range of motion to test mobility while wearing the carrier. Also, try twisting your torso to make sure that the plate carrier doesn’t shift too much to cause any discomfort.