So let's say I'm making a game in portrait mode, each scene will take the entire size of the screen, how should I size it so it takes the exact width of the screen and also work properly on different screen sizes? Also, I would like to know if there is the easier method to design the scene than selecting each individual asset on it and copy/past each one to the next.
In game settings you can set the 'screen adjustment' so it will scale to the width or height of the screen. Select a level and press 'D' to duplicate it.