You should never ride motocross without a chest protector any more than you would get on your bike without a helmet. Rocks and some can be the size of your fist, get launched or "roosted" by the wheels of your bike and if someone else roosts a rock and it hits you in the chest it can really hurt you. There have been incidents of broken collar bones and more and more often than not those injuries could have been prevented by wearing chest protection.
Now, there are two main kinds of chest protection and which one you want to wear is entirely up to you. The first type of chest protectors are heavily molded plastic that covers your chest, back, shoulders and collarbone with a heavily molded plastic form that fits over your riding jersey. This is the best protection you can get. The other type fits under your riding jersey and is called Body Armor. This type of protection fits snugly to your body and won't restrict your movement and most have vented air flow to keep you from over heating when you're riding. However, they are good, but they will not protect you to the extent that an over the jersey chest protector will.
Getting a good fit for your chest protector is easy. Just buy the same size you wear in a t-shirt for the under the jersey body armor and if you're getting a plastic chest protector that will be worn outside your jersey, it's best to get a size larger in case you're wearing a jacket in colder weather.
