I was fortunate enough to have been shown this little card trick about 18 years ago and since then have improved on the gimmick. The effect is this, you have a card selected from a normal sized deck. the card is inserted halfway in the center of the deck and a small mirror is brought out, the mirror is the same size as the width of the deck you're holding and is placed on top of the deck, so the reflection is of the back of the deck. you explain that when moving the mirror back and forth along the top of the cards the reflection suggests that the deck is getting bigger and smaller. you in fact mention that if the mirror is placed right , it'll appear as if the cards are small w/ their card still sticking out. when the mirror is picked up you have a mini deck in place of the regular deck w/ the card still in the center.
I hope I decribed this without being to confusing, I was never told the name of the effect but, it might be an early Harris effect.