Configuring a button (or any widget) in Tkinter is done by calling a configure method "config"

To change the size of a button called button1 you simple call

button1.config( height = WHATEVER, width = WHATEVER2 )

If you know what size you want at initialization these options can be added to the constructor.

button1 = Button(self, text = "Send", command = self.response1, height = 100, width = 100) 

Regarding your initial question: the button does appear physically. The problem is, since it is so large, it is hard to distinguish from the rest of the window.

Now, you said that your ultimate goal is to change the size of a button. If so, then you are on the right track: you use the height and width options for this.

However, I would recommend that you make a few changes to your code:

1. Don't make the button so huge. Even on a very big monitor, having a button be that size is way overkill.
2. Don't make the window so huge. Nobody wants an application that takes up the entire screen.
3. Use .grid instead of .place. Doing so will make it easier for you to place widgets where you want them.
4. Set the height and width options when you make the button, not after it.
5. There is no need to import sys here. Only import what you need.
6. Don't import like this: from tkinter import *. Doing so dumps a whole bunch of names in the global namespace that can easily be overwritten.

Here is my version of your script:

import tkinter as tk

def mmWindow():
    mmWindow = tk.Tk()
    mmWindow.geometry('600x600')

mWindow = tk.Tk()
# You can set any size you want
mWindow.geometry('500x500+0+0')
mWindow.title('DMX512 Controller')

wtitle = tk.Label(mWindow, text="Pi DMX", fg='blue')
wtitle.grid(row=0, column=1)

# You can set any height and width you want
mmbutton = tk.Button(mWindow, height=5, width=20, text="Main Menu", command=mmWindow)
mmbutton.grid(row=1, column=1)

mWindow.mainloop()