The documentation of QML WebView says the following:

If the width and height of the item is not set, they will dynamically adjust to a size appropriate for the content.

This seems to be the case on the Simulator, but not on device (tried with N950 and Symbian Belle). Consider the following application:

import QtQuick 1.1
import QtWebKit 1.0

Rectangle {
    width: 360
    height: 360
    WebView {
        anchors.centerIn: parent
        url: "http://www.google.com"
        onLoadFinished: {
            console.log(width)
            console.log(preferredWidth)
        }
    }
}

On the simulator it outputs the following, showing that width property got automatically adjusted:
960
0

But when run on device, the output is
0
800