Minimum screen resolution for Android devices
According to the latest Compatibility Definition Document, the definition for screen configurations require that:
- Screens MUST be at least 2.5 inches in physical diagonal size
- Density MUST be at least 100 dpi
- The aspect ratio MUST be between 1.333 (4:3) and 1.779 (16:9)
- The display technology used consists of square pixels
In practical terms, the lowest screen resolution currently being used is QVGA (240x320). There are a couple of phones that use this resolution including the HTC Tattoo and Wildfire. The smallest device I'm aware of is the 2.55" QVGA Sony Ericsson X10 Mini.
This is a device (a smart watch) with 240 x 240 resolution: http://www.omate.com
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).