Here you are:
Save it to your scripts folder, refresh your script (Filters > Script-Fu > Refresh Scripts) and you can find it under Layers > All Layers to Image Size. Works on RGB, Grayscale, and Indexed image types.
; GIMP - The GNU Image Manipulation Program
; Copyright (C) 1995 Spencer Kimball and Peter Mattis
; This program is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation; either version 2 of the License, or
; (at your option) any later version.
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
; GNU General Public License for more details.
; You should have received a copy of the GNU General Public License
; along with this program; if not, write to the Free Software
; Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
; Script can be found under Layer > All Layers to Image Size
; Define the Function
(map (lambda (x) (gimp-layer-resize-to-image-size x)) (vector->list (cadr (gimp-image-get-layers img))))
"<Image>/Layer/All Layers to Image Size"
"Resize all the layers in an image to the same size as the image."
"May 24, 2010"
SF-IMAGE "Image" 0
SF-DRAWABLE "Drawable" 0