If we look at a typical snapshot of traffic, you'll see that the sophisticated devices rule the roost. And these devices tend to have the larger screens. My Android sports a 3.7" screen, making viewing most websites A Good Experience.
What you do need is to know how your site performs on a mobile device. View your site and try navigating around it. Do those pretty menus work on a smartphone? What about that fancy Flash? Then delve into your analytics. Are your bounce rates worse for mobile devices? In short, experience your site from the POV of a mobile user of your site.