In brief
The hard roots are a good starting point; the thickness of the pieces is a separate culinary choice.
Useful to know
In radishes, look for hard roots with a smooth surface and fresh leaves if sold with them. Iowa State considers washing and preparing before slicing. Removing the leaves and ends makes it easy to form a salad portion. [1]
Thin circles are distributed differently than large quarters. For a salad with very small ingredients, you can choose a finer cut, and for a separate garnish - a larger one. It is a solution for texture and convenience, not a rule for nutritional value.
Buy relative to intended use. A large connection is not automatically a better choice if you only need a few slices. When serving, try a small portion before adding the whole amount: the specific product and taste preferences matter more than one universal number of radishes for each salad.
Sources
For further detail and to check the information: