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()

When there is only text in your button, your width and height will be based on character length, not pixel length. So, your window is defined as 600 pixels by 600 pixels, but your button is defined 15 text characters by 15 text characters, which is much more than 15 pixels :)

I added a second button and put text in both of them, exactly 15 characters, to show you exactly what's happening. More information can be found on this Tkinter Button information page.

from tkinter import *
window = Tk ()
window.title(" ")
window.geometry('600x600')

btn = Button(window, text ="X"*15, bg = "white")
btn.config(height = 15, width = 15)
btn.grid(column = 0, row = 0)

btn2 = Button(window, text = "X\n"*14+"X", bg = "white")
btn2.config(height = 15, width = 15)
btn2.grid(column = 1, row = 0)

window.mainloop()

Thanks to Bryan Oakley and finally I noticed that it is impossible to change the button height with Mac, and he suggested using label as alternative which is probably the best choice the solve the problem.

Thank you again for help.

A pixel is the smallest unit of an image, it corresponds to a how bright a single light on a monitor is (3 lights for RGB).

You can not have half an element that takes up half a pixel, as a pixel is already the smallest unit of an image.

If you have a 4x4 grid of lights you can turn them on to make a 4x4 square, as well as a 3x3, 2x2 and 1x1 (single light on) but you cant make a 0.5x0.5 square.

The width and height attributes are in units of characters (eg: 1 means the size of one average character). You can specify the width and height in units of pixels if the button has an image.

A button can have both an image and text, so a simple trick is to add a one-pixel transparent pixel as an image, which will then allow you to specify the size in pixels.

The padx and pady options also contributes to the size of the button. If you are trying to get a precise size, you need to set those to zero.

Here is a simple example:

import tkinter as tk

root = tk.Tk()
root.geometry("200x100")

pixel = tk.PhotoImage(width=1, height=1)
toolbar = tk.Frame(root)
toolbar.pack(side="top")
for size in (12, 16, 24, 32):
    button = tk.Button(toolbar, text=str(size), width=size, height=size,
                       image=pixel, compound="center", padx=0, pady=0)
    button.pack(side="left")

root.mainloop()

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) 

It's not a bug, that's just how ttk buttons work. If you need a highly configurable button, use a tkinter button. ttk buttons are less configurable on purpose. The goal of ttk widgets is to give you a set of buttons consistent with a particular theme.

Since you're on a linux system you can affect the height with pack, place or grid with appropriate options, though it's less convenient than the height option.

The height of the button is controlled both by the height option along with the borderwidth and highlightthickness. When you have three buttons stacked on top of each other you will have three times the height, plus six times the borderwidth and six times the highlightthickness. If you have a single button with a height of three, you have three times the height plus only two times the border width and two times the highlightthickness.

If you turn off the border and highlight ring, you'll see that three one-character buttons take up the same amount of space as one three-character tall button.

If you want these widgets to be really perfectly aligned with one another, it's definitely better to align them on the same grid and use the sticky argument to make the buttons stretch to fill their cell:

import tkinter as tk

root = tk.Tk()

chLabel = tk.Label(root, text="Channel group")
channelButtons = tk.Frame(root, bg='yellow')
ch1Button = tk.Button(channelButtons, text="CH1 Settings")
ch1Button.pack(fill='x')
ch2Button = tk.Button(channelButtons, text="CH2 Settings")
ch2Button.pack(fill='x')
ch3Button = tk.Button(channelButtons, text="CH3 Settings")
ch3Button.pack(fill='x')
ch4Button = tk.Button(channelButtons, text="CH4 Settings")
ch4Button.pack(fill='x')

trigLabel = tk.Label(root, text="Trigger group")
trigButton = tk.Button(root, text="Trigger Settings")

horizLabel = tk.Label(root, text="Horizontal group")
horizButton = tk.Button(root, text="Horizontal settings")

# Align the labels and buttons in a 2-by-3 grid
chLabel.grid(row=0, column=0, pady=10)
trigLabel.grid(row=0, column=1, pady=10)
horizLabel.grid(row=0, column=2, pady=10)
channelButtons.grid(row=1, column=0, sticky='news')
trigButton.grid(row=1, column=1, sticky='news')
horizButton.grid(row=1, column=2, sticky='news')

root.mainloop()