Use ipady and ipadx to add pixels inside the button (unlike pady and padx, which add pixels outside):

my_button = ttk.Button(self, text="Hello World !")
my_button.grid(row=1, column=1, ipady=10, ipadx=10)

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()

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()

Button Height: If you notice, the height of frame_1 is 30 and the height of the buttons are 100 and 20. One button height is significantly taller than frame_1. So if you maximise your tk window, you will see the height difference of the buttons. Alternatively, try setting one button height to 10 and the other to 2, and rerun your script, to see the height difference. Conclusion, the button heights can be changed.

Button Lateral Placement: The lateral placement of the buttons can be controlled by using the padx=[x_left, x_right] option of the pack system. x_left and x_right denotes the horizontal external padding to be left on each side of the button in relations to it's parent. Your can read Tk documentation for a clearer explanation on the Packer's algorithm.

from tkinter import * 
selection_window = Tk()
selection_window.wm_title("")
selection_window.geometry('{}x{}'.format(200, 150))
frame_1 = Frame(selection_window, width=200, height=30)
frame_1.pack()
Button(frame_1, text="Single",height = 10).pack(side=LEFT, anchor=S, padx=[0,40])
Button(frame_1,text="Range",command=Toplevel,height = 2).pack(side=RIGHT, anchor=S, padx=[20,0])
selection_window.mainloop()

Height: Placement:

Part 2: Per comments below, please run below script to see if changing a ttk.Button height is even possible for OSX using 'non-default' style themes and post your finding in the comment section. It worked on my Ubuntu.

from tkinter import *
import tkinter.ttk as ttk
s=ttk.Style()
print('Style themes on my system are ', s.theme_names())
s.theme_use('clam')
s.configure('bb.TButton', background='white', padding=50)
b1=ttk.Button(text='Default')
b1.pack(side=LEFT, anchor=S, padx=[0,40])
b2=ttk.Button(text='Custom', style='bb.TButton')
b2.pack(side=RIGHT, anchor=S, padx=[20,0])

padding=1 padding=40

Configuration of height is not supported in ttk.Button per Changing ttk Button Height in Python

However width is supported by ttk.Button.

I don't see where you are trying to add width in your example code, so I've taken two of the buttons (remove_button, and shuffle_button) and placed a width of 100 on remove_button.

<-SNIP->

remove_button = ttk.Button(window, text="Remove", width=100, command=remove_from_playlist)
shuffle_button = ttk.Button(window, text="Shuffle", command=shuffle_playlist)

<-SNIP->

In the future, you can see what config keys are supported for a tk widget with:

print(remove_button.config().keys())
# Supported config keys:
# dict_keys(['command', 'default', 'takefocus', 'text', 'textvariable', 'underline', 'width', 'image', 'compound', 'padding', 'state', 'cursor', 'style', 
'class'])

Additionally, for ttk widgets you can check what ttk.Style keys are supported for a specific ttk widget with:

print(ttk.Style().configure(remove_button.winfo_class()).keys())
# Supported Style config keys:
# dict_keys(['padding', 'anchor', 'width'])

where remove_button is the name of the widget you are querying. In this case, I am querying the remove_button key.

Image showing example code result