This answer is based on Rachel's answer. Her code did not work originally, but with some tweaking I was able to fix the mistakes.
import tkinter as tk
root = tk.Tk() # create a Tk root window
w = 800 # width for the Tk root
h = 650 # height for the Tk root
# get screen width and height
ws = root.winfo_screenwidth() # width of the screen
hs = root.winfo_screenheight() # height of the screen
# calculate x and y coordinates for the Tk root window
x = (ws/2) - (w/2)
y = (hs/2) - (h/2)
# set the dimensions of the screen
# and where it is placed
root.geometry('%dx%d+%d+%d' % (w, h, x, y))
root.mainloop() # starts the mainloop
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