There is lots of interest and activity in the Silicon Valley Homebrew Mobile Phone Club right now centered around building a phone with a large touch screen. I'm calling it "myPhone", and TuxPhone is the generic name for the open software and hardware components we are using. The hardware design is based on components that are openly available with freely available spec sheets, we will make our schematics, design details and cad files freely available, and its quite likely that no two phones will be built the same way. If you want it thinner and lighter, go for fewer features, a smaller battery and tweak the cad file before making the case.
The size is driven primarily by the screen, and we've settled on the screen that is used by the Sony PSP, with a touchscreen overlay added. Its a 4.3" diagonal, and is widely available, low cost. We will include a standard GSM/EDGE module, so I can just pop the SIM card from my Treo into it, and we can add GPS, WiFi, Bluetooth, Accelerometer, SDcards or hard disks, whatever custom mix we feel like. The CPU and other parts will be based on Gumstix modules.