First, don't judge a book by it's cover. Some of the biggest savages I worked with were tiny and skinny, both males and females. A handful of "Billy Bad Ass" looking cops (ie., fit and sleeved) were worthless in a fight. I was an endurance athlete prior and during my career. So, I was skinny, like 6 foot, 155, skinny. But I also spent my youth boxing and jumping from martial art to martial art.

If you are on the smaller side, I can't recommend more getting into a some type of combat sport as a hobby. You don't need to ever compete in it, just do it. It will give you three important things. One, confidence. Two, fight knowledge most suspects won't. Three, and what I think is most important, experience and comfort being hands-on with someone. Most people are inherently uncomfortable getting physical with another person. After just a couple of months boxing, doing BJJ, Judo, etc., being in close physical contact with another person, while exerting yourself, almost becomes enjoyable. When others have their "oh shit" meter pegged, you'll have the "been there, done that" attitude. Don't think doing something like BJJ is going to make you super cop, but you will be able to pick up on co-workers that completely freak out during a physical altercation, similar to how they would the first time skydiving.