Originally Posted by SilentKnight

If you constrain proportions and reduce the height to 189 - the width will only be 284. I'd be tempted to come up with a happy medium between constraining the aspect ratio and stretching it too much.

I've had banner designs in the past where the aspect ratio was outta whack with what I needed so I just redesigned it from scratch using the size I needed. Lot more work, but sometimes there's no work-around.