When a user is trying to login to my app, I am displaying a ContentDialog containing a few TextBlocks and a ProgressBar.
I choose ContentDialog because it is modal a
In Windows Phone 8.1, ContentDialog has FullSizeDesired boolean property which, when set to true, will open the dialog in full size mode. (MSDN link).You'd need to set Background to a transparent color value if desired.
FullSizeDesired
Background