In AVD Manager, you can create a new device, with the desired resolution.
EDIT: Sorry, I've don't understand your question. If you want to create a new Skin, you can copy some default skin. In the android-sdk folder, you can go to platforms\android-10 (by example), skins. Copy one folder and rename it "Sensation". In this folder, you'll find a file named "layout". Here you can define a new predefined skin with the desired resolution.
In AVD Manager, you can use the new skin.
(Sorry for my bad english)