• Make a backup:

cp /usr/bin/guake ~/Desktop/

• open guake with text editor

gksu gedit /usr/bin/guake

Then search for the line that looks like this: width = 100 - this is how the code around it looks like:

screen = self.window.get_screen()

height = self.client.get_int(KEY('/general/window_height'))

width = 100

halignment = self.client.get_int(KEY('/general/window_halignment'))

And modify the width value (percentage) to whatever you want. By default, Guake is centred but you can also modify its alignment by editing line which looks like this: if halignment == ALIGN_CENTER: and replacing CENTER with LEFT or RIGHT.