Elaboration of patthoyts' nice answer to get to an universal solution using a derived style instead of modifying the TCombobox style (but beware of a Tk bug, more on that later).

Basically, a new style with an unique name is created for every combobox (I don't know how this could scale up - maybe it's safer to apply it only where is needed). Also, combobox values are read from the widget itself and the longest one is taken: there's also a check to avoid making the popdown smaller than whe widget, if short text is inserted.

import tkinter as tkimport tkinter.ttk as ttkimport tkinter.font as tkfontdef on_combo_configure(event):    combo = event.widget    style = ttk.Style()    # check if the combobox already has the "postoffest" property    current_combo_style = combo.cget('style') or "TCombobox"    if len(style.lookup(current_combo_style, 'postoffset'))>0:        return    combo_values = combo.cget('values')    if len(combo_values) == 0:        return    longest_value = max(combo_values, key=len)    font = tkfont.nametofont(str(combo.cget('font')))    width = font.measure(longest_value + "0") - event.width    if (width<0):        # no need to make the popdown smaller        return    # create an unique style name using widget's id    unique_name='Combobox{}'.format(combo.winfo_id())    # the new style must inherit from curret widget style (unless it's our custom style!)     if unique_name in current_combo_style:        style_name = current_combo_style     else:        style_name = "{}.{}".format(unique_name, current_combo_style)    style.configure(style_name, postoffset=(0,0,width,0))    combo.configure(style=style_name)root = tk.Tk()root.title("testing the combobox")root.geometry('300x300+50+50')fruit = ['apples are the best', 'bananas are way more better']c = ttk.Combobox(root, values=fruit, width=10)c.bind('<Configure>', on_combo_configure)c.pack()c1 = ttk.Combobox(root, values=['shorter','than','widget'], width=15)c1.bind('<Configure>', on_combo_configure)c1.pack()root.mainloop()

But...

As stated before, there's a bug in Tk Combobox: the postoffest property is read only from the TCombobox style, not from derived styles.

This can be fixed by editing [python-install-dir]\tcl\tk[version]\ttk\combobox.tcl; find this line in the PlacePopdown method:

set postoffset [ttk::style lookup TCombobox -postoffset {} {0 0 0 0}]

and replace it with:

set style [$cb cget -style]set postoffset [ttk::style lookup $style -postoffset {} {0 0 0 0}]

Or, wait for my pull request to get merged and released.

The Tk library file that implements the menu posting (ttk/combobox.tcl) explicitly reads the width of the combobox widget and sets the menu toplevel to be the same width (in ttk::combobox::PlacePopdown). However you can apply a themed style to override this using the -postoffset configuration option. This exists to allow for some themes to offset the dropdown but we can set this to allow for a custom width. Note: all the widgets using this style would get the same width so you might want to derive a custom style.

In Tcl/Tk this is: ttk::style configure TCombobox -postoffset {0 0 300 0} to setup the dropdown to 300 pixels wide (x y width height).

UPDATE

The following python tkinter code adds some code to the <Configure> event to get the size of the combobox and update the postoffset width such that the dropdown will match the size of the first string in the list of strings. The result looks like this:

import tkinter as tkimport tkinter.ttk as ttkimport tkinter.font as tkfontdef on_combo_configure(event):    global fruit    font = tkfont.nametofont(str(event.widget.cget('font')))    width = font.measure(fruit[0] + "0") - event.width    style = ttk.Style()    style.configure('TCombobox', postoffset=(0,0,width,0))root = tk.Tk()root.title("testing the combobox")root.geometry('300x300+50+50')fruit = ['apples are the best', 'bananas are better']c = ttk.Combobox(root, values=fruit, width=10)c.bind('<Configure>', on_combo_configure)c.pack()root.mainloop()

We do this using the event binding as this ensures we can get the actual size of the widget on screen to remove that width from the offset.

Late to the party, but I encountered this issue recently...

I tried dipanda's solution, and confirmed that the derived styles were being altered during the <Configure> event, but for some reason the postoffset option wasn't having any effect. It was being set, but the pop-down remained the same width as the original widget.

What worked for my purpose was the following; pretty similar to dipanda's code:

def combo_configure(event):    combo = event.widget    style = ttk.Style()    long = max(combo.cget('values'), key=len)    font = tkfont.nametofont(str(combo.cget('font')))    width = max(0,font.measure(long.strip() + '0') - combo.winfo_width())    style.configure('TCombobox', postoffset=(0,0,width,0))

But here's where I went weird:

Instead of binding this to the <Configure> event, I instead bound it to the <ButtonPress> event:

widget.bind('<ButtonPress>', combo_configure)

I acknowledge that the changes affect all Comboboxes in the window, but since you can only have one dropped down at a time, it doesn't really matter; it's transparent as far as the user is concerned. Plus, no need to modify the combobox.tcl file!

Just my $1.05...