That should work too, the HUD BP is just a place I know that that node is easily grabbed. That should return the same values as the Event Receive Draw HUD but it will only do the check the one time, whereas the HUD BP will do the check every frame so on a mobile device at beginplay the resolution may be different than a few moments later if the player rotates their screen. You can test this using just the PIE, run a print string from the HUD BP size x and y and then go from the editor viewport to full screen mode and you will see the return values change.