Basses come in multiple sizes including 1/4, 1/2, 3/4, and 4/4.
This size nomenclature creates some confusion as a 1/2 bass, for example, is not really half the size of a 4/4 bass. As you can see by the chart below, the sizes are actually only slightly different.
The various sizes accommodate the player's height, arm length, and hand size; it's more comfortable to play a bass that's your size and your hand more easily reaches the notes.
Unlike violins where a violinist typically grows into a 4/4 violin, with the bass, the standard and most popular size by far is 3/4. Few 4/4 basses are made or played.
One of the biggest reasons for this is that the bass requires long stretches with the hands. Most bassists feel that the difference in sound between 3/4 and 4/4 isn't enough (indeed many prefer the 3/4 sound) to justify the extra lengths that must be traversed.