Luke Wroblewski works in the field of micro mobile interactions and is currently involved in an Internet start-up. Respected speaker, designer and ideas person he is also considered an expert on responsive web design. Check his website here.
Here is a great video of his where he discusses screen sizes and how to adapt to them when designing a responsive website.
This video is also of value to folk who are still a little confused as to what responsive and mobile friendly is all about.