You'll struggle to find any jacket that is fully waterproof and 'breathes' and the Core range is the more affordable collection so won't be as highly technical, I'd recommend wearing some kind of long sleeve under it to keep the material away from the skin and try not to be too concerned with breathability. As for the fit, if there's much sagging / bunching when in the riding position, then I'd consider it too big. However if you're more comfortable with a roomier fit, then ride that.
You'll struggle to find any jacket that is fully waterproof and 'breathes' and the Core range is the more affordable collection so won't be as highly technical, I'd recommend wearing some kind of long sleeve under it to keep the material away from the skin and try not to be too concerned with breathability. As for the fit, if there's much sagging / bunching when in the riding position, then I'd consider it too big. However if you're more comfortable with a roomier fit, then ride that.