Small Mario is one of Mario's Power-Ups (or, Power-Downs). Mario cannot access this form by using an item, but by being hit by foes.
Small Mario doesn't have an ability. It rather serves as a starting form in 2D platformers. When hitting by an enemy, Small Mario dies. That's why he need a Super Mushroom to grow to Super Mario.
- A Poison Mushroom will make Super Mario to Small Mario, since it serves as a bad guy. This is the only time Mario'll turn into Small Mario by items.
Small Mario riding Yoshi in New Super Mario Bros.: Twist It!
Small Luigi in Mario 3D World.