Look up the screen resolution for the phone you are using, in this case it is a Samsung Galaxy S6.
Then go into Unity and click inside the "Game" window at the top left corner.
Then click the + icon.
Fill in the proper sizes in the two spaces provided, ie Width and Height. Label this anything you want, but you might want to name it something like this:
Now you have a scene where you can accurately place your sprites and not have to worry about size issues later on.
Keep on Kraken,