Does anyone know, or know how to determine, the dimensions of the Samsung Galaxy S6+ screen in pixels? The reason I want to know is because I want to make a wallpaper for my girlfriend that exactly fits the screen
Samsung devices allow, under Developer Options, the ability to mark screen locations.

See Settings > Developer Options > Input

Select
Show Touches,
Show pointer location

Move to the points desired, and note their coordinates.

If you find the constant coordinates undesitreable, turn OFF the options after the operation is complete to your satisfaction.

To turn on Developer Options if needed:


Go to app drawer and open Settings
Go to bottom and tap About device
Tap repeatedly Build number 7 times. Starting at 3rd tap, you'll see a toast saying You are now X steps away from being a developer.