Usual size for D6 dice?
This page on DiceGameDepot has an exhaustive article about it. A visual aid from that article:



From my experience and the info from article, the 16mm die is the most common plastic die, used in the most packaged tabletop games, likely for it's good balance between readability and usability, while taking up fewer space in boxes or storage than the bigger ones.
Smaller dice are often hard to roll because of their lower weight, while larger dice more easily roll off the table and are louder.

From a player's perspective, I feel that the 19mm die looks and feels better, especially in direct comparison. Using 16mm dice instead is likely a business decision. For some corner cases, such as marking counters on Magic: The Gathering cards, I prefer 12mm dice, as you don't need to roll them, they obscure the card less, and it's easier to carry large quantities of them.

As for wood dice, which ship more frequently than plastic dice in older German tabletop boxes that I own, those have 15mm, 18mm and 30mm lengths after a quick measure. I wasn't able to find any sources on the numbers there, but I've been able to come up with 15mm dice much more often than any other within my collection.