The diameter is an important factor to consider when replacing your vehicle's original wheels. The distance measured is not from the face of the wheel as commonly thought, but instead from the bead seats where the tyre fits onto it. To ensure your vehicle's speedometer reading is not affected, it is important that the overall rolling diameter of your vehicle's tyres remain the same. So any increase in wheel diameter must result in a reduction of tyre sidewall height to maintain the same overall rolling diameter.
Upgrading to a larger diameter wheel will dramatically improve the look of your vehicle and improve handling responsiveness, whilst also seeing a slight reduction in ride comfort due to the lower profile tyres required to maintain the overall rolling diameter.
Alternatively, selecting a smaller diameter wheel will improve the ride comfort of your vehicle, at the expense of some handling responsiveness and visual asthetics. It is also important to check whether the smaller diameter wheels will clear your brake calipers.