Autosizing the pixel size of text based on text length or size using fontSizeMode

I am trying to use fontSizeMode in order to fix a large text in a rectangle.

import QtQuick 2.9
import QtQuick.Window 2.2

Window
{
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    
    Rectangle
    {
        color: "Red"
        height:50
        width:50

        Text
        {
            text: "Hello"
            fontSizeMode: Text.Fit
            minimumPixelSize: 5
            font.pixelSize: 50 
        }
    }
}

My aim is to shrink the text size if the text is bigger than the rectangle, and a minimum pixel size is given as shown in the program. But the text is not shrinking.

How can I solve this problem?

Use width:parent.width and height:parent.height:

Window
{
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    
    Rectangle
    {
        color: "Red"
        height: 50
        width: 50

        Text { 
            width: parent.width
            height: parent.height
            text: "Hello"
            fontSizeMode: Text.Fit
            minimumPixelSize: 5
            font.pixelSize: 50 
        }
    }
}