The building blocks come in four sizes.
Maxi size, introduced in 1985, intended for very young children. The blocks feature slightly rounded corners and edges and have tall rounded studs.
Mini size, introduced in 1989, designed for toddlers and preschoolers. The bricks likewise have slightly rounded edges and corners. The studs are 5/12 the height of the body of the brick. This size is the same size as Duplo.
Micro size, introduced in 1991, has sharp edges and corners, and the studs are 1/6 the height of the body of the brick. This size is the same size as ordinary LEGO bricks.
Nano size, introduced in 2004, is the smallest.