[racket] get-slides-as-picts dimensions

I hacked up this short program to fade between slides. It seems to work well (although it won't work for animations) but I'm wondering how to get rid of the
hard coded size constraints (640, 480). I just want to generate slides that would be the normal size they would have looked like if I was running x.rkt.

#lang slideshow

(require slideshow/slides-to-picts
         slideshow/play)

(define slides (get-slides-as-picts "x.rkt" 640 480 #f))

(let loop ([slides slides])
  (match slides
    [(list slide1 slide2 rest ...)
     (play (lambda (n)
             (fade-pict n slide1 slide2))
           #:steps 5)
     (loop (cons slide2 rest))]
    [(list last)
     (slide last)]
    [(list) (error 'slides "expected at least one slide")]))