Calligraphy Brushes come in different sizes. The main categorization is numerical from 1 to 10. Brush of a sizing number 1 has a diameter of approximately 2.1cm at the base of the tuft (near the brush axis), while number 10 has 0.5cm, although depending on the producer those measurements may differ slightly.
On top of that there are much smaller brushes as well as much bigger to gigantic ones. Also, the hair length can differ drastically, from a few millimetres to tens of centimetres. Consequently, the brush tuft can be short, medium, long or extra-long, as well as thin and long, or thick and short, etc.
They all have different applications in calligraphy. For instance short hair brushes are more suitable for standard script (楷書、kaisho) or clerical script (隷書, reisho), as they offer more control over the brush tip. A mid-size brush is still good for standard script, but especially suitable for semi cursive (行書, gyousho) and seal scripts (篆書, tensho). The long one is often used for cursive (草書, sousho) and semi-cursive scripts, as well as kana (かな). The extra-long brush is perfect for avant-garde calligraphy (前衛書道) and modern poetry.
This however does not mean that a long soft brush cannot be used for clerical script or standard script. Some calligraphers purposely study styles requiring precision with difficult to handle brushes to further master their skills.