Thanks, 

This works, but the links you posted are for windows, form mac there are  oly 3 possible values for THUMBSIZE:

https://knowledge.autodesk.com/support/autocad-for-mac/learn-explore/caas/CloudHelp/cloudhelp/2019/E...

But despite of this, I can set other values over 2, as you described.

The thing I can't undestand is: 

if the thumbs are saved in the DWG file, why after migration from Autocad 2017 to Autocad 2019 all the thumbs of my files had a low resolution? I haven't re-saved them with Autocad 2019...