This page suggests needles suitable for various applications according to what fibers you are using and what stage your project is at. It should be noted that there are a range of characteristics within each fiber type, so choice involves a degree of personal preference. This information is intended as a starting point. As your experience increases, your ability to choose appropriate needles for each task will increase.
Use for thicker and rougher fibres, when durability is an important consideration. Vessels will be more sturdy made with course wool. Colors will penetrate each other and mix.
Use for most projects. Medium wool will react quickly and forgive inaccuracy so you will have more time for creativity. This fibre is a good choice for beginners.
Use for small or tiny projects, for a clear definition of fine details and for a fine surface finish. Harder to work with than medium thickness fibres, but ideal for jewellry.