from tkinter import *

def resize(ev=None):
    label.config(font='Helvetica -%d bold'%scale.get())

top=Tk()
top.geometry('250x150')
label=Label(top,text='Hello World',font='Helvetica -12 bold')
label.pack(fill=Y,expand=1)
scale=Scale(top,from_=10,to=40,
            orient=HORIZONTAL,command=resize)
scale.set(12)
scale.pack(fill=X,expand=1)
quit=Button(top,text='QUIT',command=top.quit,activeforeground='white',activebackground='red')
quit.pack()
mainloop()