There is no single, exactly correct way or exact formula. It all depends on many factors: starting with the thickness of the musician's fingers, the style of music, psyche, or the type of temperament of the performer himself.
The only true statement may be that playing with a pick should be convenient, comfortable and enjoyable.
Let's highlight a few basic parameters. First, it is material, from which the mediator is made. It can be different: plastic, metal, bone.
Plastic picks are very common because the material itself is very cheap. Care should be taken to ensure that the plastic pick does not slip in your hand when you play, otherwise the risk of falsehood increases. The surface of the pick must be roughened to avoid this unpleasant slipping. This applies not only to plastic picks, but to everyone else. It is also necessary to remember that the negative feature of plastic is that when you play it is erased on the strings and soon you will have to buy a new one.
Metal picks are more wear resistant than plastic picks. They also have a unique sound when played. However, metal picks increase the rate at which the strings wear out! And you run the risk of breaking your string at the most inappropriate moment for this.
Bone mediators are rare. Their cost, respectively, is much more expensive than that of the previous types of mediators. But by purchasing a bone pick, you get wonderful timbres and mesmerizing sound.
The second important parameter when choosing a pick is his thickness... Basically, the thickness of the pick is from 0.38 mm., And up to 3 mm. Thick picks are suitable for high-speed playing and mind-blowing solos. Thin picks - for playing rhythm parts when using acoustic instruments.
The size and the form mediators play an important role too. The choice of these parameters of the mediator is based on personal psychophysiological qualities (length and thickness of fingers, hand size, temperament and character of the musician).
So, we have considered several main indicators that you should pay attention to when choosing a mediator. But the most important thing is personal feelings when choosing one or another mediator. You should experience comfort, convenience, a sense of complete satisfaction, confidence and positive emotions from the sound you generate.