This pack turns you into Ant Man
Just go into your inventory. Press the player head button so that you morph into Ant Man and gain his suit.
Press the X in your inventory to unmorph.

As Ant Man, you can right-click in any slot in your inventory. You then have options to shrink or grow.

When you are tiny, mobs will not be able to see you.
When you are giant, you will be about 6x taller.

Make sure you have the resource pack.
Also, this requires Optifine to load some textures.