Not too familiar with the
stage version, (only saw the movie) but depending on your
venue, you could do a few different things. The one that I see as being the easies, is making some sort of a movie almost, and somehow project the visual onto a
screen of some sort for the audience (if you have reallyyy good media arts people. if not i suggest avoiding that because if it doesn't look impressive, it will totally take away from your show).
Maybe some sort of a silhouette
effect with some cool light changes. Paper mache, to me, sounds like it has the potential to look tacky.
An inflatable has potential, maybe a weather balloon? No matter what it is you inflate, I think youd be best off to just do a little
bit of the "growing" so the audience understands that its getting larger and larger, and then going to a few lighting changes, ending with a
blackout and then turing on the lights, inside the peach, or rather outside of a pre-built huge peach.
Also, maybe even simply making it out of wood, doing a really nice paint job, and making several of them, getting larger and larger in size, and just change out the pieces in between blackouts and light changes.
(Sorry if a lot of that didnt make sense. if anything seems like it could be a possibility for you, and you want me to clarify Id be more than happy to!)