Python 2, 61 bytes

from Tkinter import*
print 'x'.join(map(str,Tk().maxsize()))

For single display setups, this matches the output from the site. This gives entire resolution for multiple displays.

Python 2, 61 49 bytes

Thanks @Jonathan-allan, @felipe-nardi-batista

from Tkinter import*
print'%sx%s'%Tk().maxsize()

For single display setups, this matches the output from the site. This gives entire resolution for multiple displays.