These are Ben Wa Balls for sex play or exercice.
Diameter is 1 inch 2/3 (4cm), it is generally a good size for vaginal use. It is also nice for anal use with a little experience.
It is recommanded to use it with a little cord to facilitate the dildo removal.
Materials options :
While ABS or Nylon are a safe materials, 3D printing make it microporous. For hygiene reasons it is necessary that you coat it with a food grade resine. This can be done following this tutorial : https://3dprint.com/36851/sexshop3d-safe-sex-toys/
Plastic can also be used for making a mold an replicating the Dildo in Silicone. This option allow the production of several dildos and the soft touch of Silicone. You can find several tutorials on internet. For example this video : https://www.youtube.com/watch?v=99uGtFyEGyE
Width : 9.9 cm
Height : 4.2 cm
Depth : 4.2 cm