Android imageview width percentage

App Type: Android SDK: 3.2.0 Device: Galaxy S3 i9300 Version: Android CM 4.3.1 Host: OSX 10.9.1 Alloy

Just added imageview to xml:

<Alloy>
    <Window>
        <ImageView image="/images/logo.png" id="logo"></ImageView>      
    </Window>
</Alloy>

"#logo": {
    width: "50%"
}

ps: if I put the height, it works but I can't go trough this way, the parent size will change from device to device