It’s easiest to start with your collar for sizing and work your way down to the belly plate. The collar should almost (but not!) be choking you with the outside edges ending between the balls of your shoulders. From there, your chest plate edges should line up to the outside of your collar plate. The belly plate typically ends around your belly button (slightly above, at, or slightly below) - it all depends on the person and their body type.
Without seeing the collar plate taped on, I’m uncertain if these plates are too small or about the right size. My gut tells me these are on the small side, but again I’m uncertain without the collar plate. If you can tape that on in addition to the other plates, that would greatly help!
A great resource for building is getting in contact with your local clan as well as your clan’s ruusalor solyc; your clanmates and officers should be able to give you hands on help at armor parties
