It’s important to remember that responsive design was a response to evolution in phone and tablet technology. It’s not a static request. We came upon this clear, helful chart by Paintcode that breaks down the various sizes that are involved in different iPhones.
And this is just the iPhone. There are similar charts for Android phones, iPads and desktop/laptops. We enjoy the challenge and like to keep our current and future clients informed on what goes into requests such as these. After all, responsive is not just a design and development topic. Organizations have to make sometimes difficult content choices when planning for different devices.
For more perspective on this topic, you can also check out this illustration that conveys numerous screen variations for the desktop. It underscores how the notion of “pixel perfect” is fading from importance as questions such as “which pixels?” and “on which screens?” come to the fore.