An example describes the problem best:

main.qml

import QtQuick 1.0

Rectangle {
    width: 400
    height: 40
    TestButton {}
}

TestButton.qml

import QtQuick 1.0

Rectangle {
    width: 40
    height: parent.height
}

When opening main.qml the used TestButton should have a height of 40.
The height is 0 instead. This is a bug.