19s are fine, they've been standard fitment to Sline models for a while now.
My previous Sline had 18s which were also fine, but I don't think there is much difference in ride quality between the two, a little firmer with 19s. But 19s look more suited to the vehicle in my opinion.
They don't fill the wheel arches any more than 18s because the rolling diameter is fundamentally the same, its just there's less tyre and more alloy!