1. You can grow yourself and any object you touch. The level of strength and durability yourself and objects increase to depends on size they grow to. You can grow select portions of your body. But, the object in question will only stay enlarged for about an hour. You can only grow up to 20ft, growing too large, too fast is painful.
2. You can shrink yourself and any objects you touch. The level of strength and durability yourself and objects increase to depends on the size they shrink to. You can shrink select parts of your body (it might loook weird though). But, the object in question will only be shrunken for about an hour. You could only shrink down to 1 inch. Shrinking too small, too fast is painful.
For convenience sake, your clothing grows or shrinks along with you. Your body is able to withstand the strain of growing and shrinking without destroying itself, you basically ignore the square cube law. At max size for both, you reach Strength/Durability level of Supernatural type 1. So yes, you could survive being stepped on.