The Narrow (N) flag makes your tank be incredibly thin from the front but you'll be normal size from the side. Even though narrow tanks are super thin, it's not impossible to be shot from the front.
Having a very thin tank is helpful when it comes to dodging shots even when they're tightly grouped in a spray. While carrying the Narrow flag, it's in your best to fight your enemies head on where it's harder to hit; avoid turning too much making your regular sized side an easier target.
Maps may have secret areas available through tiny gaps intended for Narrow tanks to be able to fit through. Be careful with these small gaps because they put you at risk with dangerous ricohets that are incredibly difficult to dodge, making you a very easy target.