In brief
Small and medium roots are a convenient starting point for choosing a gentle texture.
Useful to know
Iowa State recommends in turnips to look for small to medium-sized, hard roots. The turnips description includes a white surface with a possible purple top. This is a different material from black radish and should not be automatically carried over all products with the word "turnip". [1]
When cooking, first specify what the recipe means. The translation of the name is sometimes less accurate than the product description. The similar shape does not mean the same aroma or the same behavior of each species.
For purchase, compare hardness and general condition, instead of choosing only by size. If you are going to prepare a mixture of root crops, the slicing should follow the specific method. There is no universal size of the cubes that provides the same result at every tray, quantity and temperature.
Sources
For further detail and to check the information: