Many Nokia phones have screen size 128x128, but the emulator has a larger screen, so can I set the emulator's screen size by myself? and, the midp provides functionalities to load images/audio, but if i write a game, i may need to load a map file that is generated by my mapeditor, so how to do this? I don't believe those role-play game's maps are hardcoded.
If you are using the Wireless Toolkit from Sun you can also change the properties of a particular device.
In the folder where you installed the Wireless Toolkit there is a folder called 'wtklib\devices'. There you have the folders of all devices (skins) that the emulator uses. In the folder of each of those devices (say 'DefaultColorPhone') you have a properties file (DefaultColorPhone.properties).
In that file you can set the screen size and many other properties.