First we’ll look a the file that wraps the Plasma5 widget’s config page. The file is called AppletConfiguration.qml in the plasma-desktop repo.

Since the main item in the this file is called id: root, my first attempt was to log it in my config page.

Item {
	id: page

	Component.onCompleted: {
		console.log('root', root)
	}
}

However, unexpectedly, it logs the StackView instead of a AppletConfiguration instance.

qml: root StackView_QMLTYPE_195_QML_201(0x5620e8559fd0)

If we try logging console.log('mainColumn', mainColumn) we get:

qml: mainColumn QQuickColumnLayout_QML_199(0x5620e84c02b0

And if we try logging the parent item of mainColumn like with console.log('appletConfiguration', mainColumn.parent) we get:

qml: appletConfiguration AppletConfiguration_QMLTYPE_196(0x5620e850ec00)

Now that we’ve gotten a way to reference the main item of the config window, we could resize the window. Or we could hide the keyboard shortcut tab.

Item {
	id: page

	Component.onCompleted: {
		// https://github.com/KDE/plasma-desktop/blob/master/desktoppackage/contents/configuration/AppletConfiguration.qml
		var appletConfiguration = mainColumn.parent
		appletConfiguration.width = 1400

		// Remove default Global Keyboard Shortcut config tab.
		var keyboardShortcuts = appletConfiguration.globalConfigModel.get(0)
		appletConfiguration.globalConfigModel.removeCategoryAt(0)
	}
}

Please note AppletConfiguration.qml is private code, not a public API, so it may change without notice.