Please, what is the device with the minimum screen resolution on the market?
Take a look at Supporting Multiple Screens to see all of the possible screen resolutions, and Screen Sizes and Densities to see what the current Android market looks like in terms of both measurements.
Right now a "Small Screen with Low Density" has an expected resolution of 240x320, so you can think of that as the market's minimal screen resolution for now (until Google updates these figures).