How to set QWidget size to match that of tab parent in PyQt5

Question

I'm trying to make some tabs in my GUI to have a background image cover the whole area. I invoke the general GUI using the following:

self.tabs_widget = LayoutWidgets(self)
self.setCentralWidget(self.tabs_widget)

Afterwards, I use the following code to make some tabs and child widgets.

class LayoutWidgets(QWidget):

    def __init__(self, parent):
        super(QWidget, self).__init__(parent)

        layout = QVBoxLayout(self)

        self.tabs = QTabWidget()
        self.myUploadProgress = QProgressBar()
        layout.addWidget(self.tabs)
        layout.addWidget(self.myUploadProgress)

        tabNames = ['Tab 1', 'Tab 2', 'Tab 3']
        self.tabsIndex = []
        for inx, tabName in enumerate(tabNames):
            self.tabsIndex.append(QWidget())
            self.tabs.addTab(self.tabsIndex[inx], tabName)

        self.masterTabLayoutIndex = []
        self.masterTitleLabelIndex = []

        masterFont = QFont()
        masterFont.setPointSize(14)

        self.tabsIndex[0].setStyleSheet("background-color: blue")
    
        print(str(self.tabsIndex[0].frameGeometry().width()))
        print(str(self.tabsIndex[0].frameGeometry().height()))

I end up getting this as the output. This is fine and all, but the width and height of the tabsIndex[0] widget ends up being 640 and 480 in the print statements.

This doesn't seem to match what I'm seeing.

Also, as I try to add widgets such as labels, I am unable to have them span the whole width and height of this tab section.

How can I add a widget and size it in such a way that it takes up the whole width and height? The ultimate goal is to have a background image fit nicely in the border of the tab, and a few buttons placed on top of the image at certain spots.

Thanks