The Amazing Enlarging Machine
This needs a bit of advance preparation in collecting props but it's well worth it. The basic idea is the the presenter introduces her "Amazing Enlarging Machine" to the audience - it may look exactly like a blanket held up by two volunteers, but a label may convince them.
The audience is encouraged to sit as close as possible to the "machine" because of the extreme small size of some of the things being used. With a lot of preamble the presenter throws a teaspoon over into the machine, and after lots of bang-whizz-wheeee noises off, the assistant hidden behind the blanket throws out a ladle or large spoon, proving that the Amazing Enlarging Machine does indeed work.
Carry on "enlarging" anything you can get a big/little version of - trainer liner sock / hike boot sock, little strappy top/big baggy tee-shirt, single tissue/box of tissues - just use your imagination.
The final crunch comes when the presenter throws a thimbleful of water into the machine : where you go from there depends on where and when you are doing the sketch - from a full bucket if outdoors and in hot weather to torn paper confetti if indoors and you don't want to end up mopping the floor forever! (It still has a good squeal factor, though!)