#Processing 3, 37 bytes

fullScreen();print(width+"x"+height);

fullScreen() causes the app to launch with the maximum dimensions - the display resolution. One byte less than the obvious

print(displayWidth+"x"+displayHeight);

Processing 3, 37 bytes

fullScreen();print(width+"x"+height);

fullScreen() causes the app to launch with the maximum dimensions - the display resolution. One byte less than the obvious

print(displayWidth+"x"+displayHeight);