Based on this script: http://www.gimptalk.com/index.php?/topic/2347-command-line-script-fu-to-scale-an-image/ This'll do non-destructive resizing.

batch-scale.scm

;; Routine scales images based on the two parameters
;; if x-factor > 0 then image width is scaled to that width (if y-factor==0 then keep aspect ratio)
;; if x-factor < 0 then image width is scaled by magnitude of that amount (y-factor==0 means flip horizontally)
;; if y-factor > 0 then image height is scaled to that height (if x-factor==0 then keep aspect ratio)
;; if y-factor < 0 then image height is scaled by magnitude of that amount (x-factor==0 means flip vertically)
;; if x-factor AND y-factor = 0 then rotate image 90 deg CCW

(define (batch-scale-image pattern x-factor y-factor suffix)
  (let* (
      (filelist (cadr (file-glob pattern 1)))
      (width)
      (height)
      (flip-horiz FALSE)
      (flip-vert FALSE)
      (extension "png")
      (basename)
      (fullname)

      )
    (while (not (null? filelist))
      (let* (
          (filename (car filelist))
          (image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
          (drawable (car (gimp-image-get-active-layer image)))
          )
        (cond 
          ( (< x-factor 0)
            (set! width (* (car (gimp-image-width image)) (abs x-factor)))
            )
          ( (> x-factor 0)
            (set! width x-factor)
            )
          ( (= x-factor 0)
            (cond
              ( (> y-factor 0) 
                (set! width (* (/ y-factor (car (gimp-image-height image))) (car (gimp-image-width image))))
                )
              ( (= y-factor 0)
                (set! flip-horiz TRUE)
                (set! width (car (gimp-image-width image))) 
                )
              ( (< y-factor 0)
                (set! flip-horiz TRUE)
                (set! width (* (abs y-factor) (car (gimp-image-width image))))
                )
              )
            )
          )
        (cond 
          ( (> y-factor 0)
            (set! height y-factor)
            )
          ( (< y-factor 0)
            (set! height (* (car (gimp-image-height image)) (abs y-factor)))
            )
          ( (= y-factor 0) 
            (if (<= x-factor 0)
              (set! flip-vert TRUE)
              )
            (set! height (* (/ width (car (gimp-image-width image))) (car (gimp-image-height image))))
            )
          )
        (gimp-image-scale-full image width height 3)
        (if (and (= flip-horiz TRUE) (= flip-vert TRUE))
          (gimp-image-rotate image ROTATE-270)
          (begin
            (if (= flip-horiz TRUE)
              (gimp-image-flip image ORIENTATION-HORIZONTAL)
              )
            (if (= flip-vert TRUE)
              (gimp-image-flip image ORIENTATION-VERTICAL)
              )
            )
          )


        (set! basename (car (strbreakup filename ".")))
        (set! fullname (string-append basename "." suffix "." extension))

        (gimp-file-save RUN-NONINTERACTIVE image drawable fullname fullname)
        (gimp-image-delete image))
      (set! filelist (cdr filelist))
      )
    )
  )