The Label widget doesn't line-wrap. The Message widget will line-wrap text, but forces it to be roughly square. Here's an example:

from Tkinter import *

root = Tk()
root.title("hello")

Message(root, text=48*'xxxxx ').grid(row=0, column=0, columnspan=3)

Label(root, text='Name:').grid(row=1, column=0)
Entry(root, width=50).grid(row=1, column=1)
Button(root, text="?").grid(row=1, column=2)

Button(root, text="Left").grid(row=2, column=0)
Button(root, text="Center").grid(row=2, column=1)
Button(root, text="Right").grid(row=2, column=2)

root.mainloop()

I know that I can use aspect=700 to change the shape, but hard-coding numbers like that is what I'm trying to avoid.

wraplength is indeed the correct answer, but it's a fixed width. If you have a dynamically sized Label you will need to update wraplength as it is resized. You can do this by binding the <Configure> event:

label = tk.Label(root, text="My long text")
label.bind('<Configure>', lambda e: label.config(wraplength=label.winfo_width()))

In some configurations I do get wrapping that is slightly off, i.e. the text runs slightly off the edge of the Label before wrapping. I haven't figured out whether that's due to the font, any margins/padding, or what is going on. I just worked around this by subtracting a fixed amount from label.winfo_width() before feeding it into wraplength.