If it’s only for when your app starts, you should try the Activated event.
Otherwise, use a timer with a short period that you enable when the window resizes. The timer then can handle the new size.
Even without implicit instance, the thing is to avoid resizing/setting to full screen from the opening event if the goal is to query the final size, as the resize will happen after the event.
Well actually the goal is just to adjust the controls to different touchscreen sizes.
The application itself is always set to fullscreen, so Screen(0).Width/Height seems to be the simplest way as in fullscreen mode this represents the window size when to menu is used.