From the Super Mario Wiki
Shrinker Pipes are objects that appear in Super Mario 64 and its remake, exclusively in the level Tiny-Huge Island. Though they look visually similar to Warp Pipes and use the same sound effects, they do not transport the player anywhere. Instead, they change the user's size, either making them much larger or much smaller than their surroundings. Using a Shrinker Pipe also resets certain events, such as the race with Koopa the Quick, allowing them to be re-attempted without exiting the stage.
There are three Shrinker Pipes on Tiny-Huge Island. One is located near the level's start, on the same platform as the Fire Piranha Plants. A second is located at the base of the mountain, above the source of the Big Steelies. The third is located on the opposite corner of the island from the level's start, at the point where the race with Koopa the Quick ends.