Which page dimensions should i use for develop an app for ET1??

There is a discussion about detecting screen dimensions here

http://motorola-dev.hosted.jivesoftware.com/message/1095#1095

The screen dimensions of the ET1 are 1024x600px. If you are wanting your application to be easily portable between devices with different screen dimensions (now, or in the future), I would recommend specifying the dimensions of your visual elements in ems. This will make it very simple to resize for different screen sizes. I will be making a video tip on this topic next week.

An interesting post on ems:

http://mondaybynoon.com/2006/03/13/effective-style-with-em/