Can I adjust the size of message box created by tkMessagebox?

As far as I’m aware you can’t resize the tkMessageBox, but if you’re willing to put in the effort you can create custom dialogs.

This little script demonstrates it:

from tkinter import * #If you get an error here, try Tkinter not tkinter

def Dialog1Display():
    Dialog1 = Toplevel(height=100, width=100) #Here

def Dialog2Display():
    Dialog2 = Toplevel(height=1000, width=1000) #Here

master=Tk()

Button1 = Button(master, text="Small", command=Dialog1Display)
Button2 = Button(master, text="Big", command=Dialog2Display)

Button1.pack()
Button2.pack()
master.mainloop()

When you run the script you should see a master window appear, with two buttons, upon pressing one of the buttons you’ll create a TopLevel window which can be resized as is shown in the script designated by#Here.
These top level windows act just like standard windows and can be resized and have child widgets.
Also if you’re trying to pack or grid child widgets into the TopLevel window then you’ll need to use .geometry not -width or -height, this would go something like this:

from tkinter import *

def Dialog1Display():
    Dialog1 = Toplevel()
    Dialog1.geometry("100x100")

def Dialog2Display():
    Dialog2 = Toplevel()
    Dialog2.geometry("1000x1000")

master=Tk()

Button1 = Button(master, text="Small", command=Dialog1Display)
Button2 = Button(master, text="Big", command=Dialog2Display)

Button1.pack()
Button2.pack()
master.mainloop()

Hope I helped, try reading up on the TopLevel widget here:
https://dafarry.github.io/tkinterbook/toplevel.htm