320 × 480
The really bad news: Although all of these browsers run on WebKit, all of them run WebKit just a little differently. Peter-Paul Koch of QuirksMode ran 19 WebKit-based browsers through 27 tests and described the results as "thinly veiled chaos."
Yes, there are a lot of devices, but you don't have to support them all. Do so by eliminating the outliers:
* Sorry, RIM. Sorry, Microsoft. You had your day in the mobile market (well, RIM did; Microsoft never did quite figure out a solution to Windows + mobile device + usable), but it's over. There may still be plenty of these devices, but research has shown that they aren't used to surf PDF.
* Sorry, small screens. Unfortunately, a 240-by-320 surface is just too small for a good browsing experience. The Pixi's 320-by-400 screen is almost twice as large, yet barely made the cut.
* Sorry, third parties. Every device you should care about ships with its own Web browser. If you have an Android, you have a choice of browsers, including Opera Mini and Mobile Firefox. But you have to draw the line somewhere, and unless you have a fully funded lab, it's unlikely you'll be able to test your site on every permutation of browser and screen size. So stick with the browser that the device maker included.
Anything else you're thinking about dropping? This is where your server logs come in handy. Keep in mind, though, that while you may have few mobile visitors now, it might be solely because they don't (yet!) find your site welcoming.
The nitty-gritty of making your site over