What is the name of the item that makes Mario grow in size?

The item that makes Mario grow in size is the Super Mushroom.

When small Mario collects a Super Mushroom, he changes into his larger “Super” form. In many games, this transformation also lets him break certain blocks and gives him an extra layer of protection: taking damage usually shrinks him back to small Mario instead of costing a life immediately.

The power-up first appeared in the original Super Mario Bros. and became one of the series' defining symbols. Its design resembles a red-and-white spotted mushroom, visually echoing the real fly agaric, Amanita muscaria. Shigeru Miyamoto has explained that the grow-and-shrink mechanic emerged partly because early testing made Mario seem too tall, inspiring a way to change his size during play.

The other options have different functions. A 1-Up Mushroom grants an extra life, a Starman provides temporary invincibility, and a Fire Flower lets Mario throw fireballs. The Mega Mushroom can also make Mario enormous in later games, but the standard item and canonical answer for ordinary size growth is the Super Mushroom.