Interesting update. I actually tried adding back size attribute to a field in V15 and it works without issue or any warning in the log! I was surprised as I took the advice from an OCA article https://github.com/OCA/maintainer-tools/wiki/Migration-to-version-14.0.
However I can see that even in V16 there are still many uses of the size attribute used on char fields, for example:
Anyone know if Odoo backtracked on this plan and it's no longer relevant?
HI Chris Mann,
Second option will be try to add constraints and check length of the char field in its method. You can raise exception for the size exceed.
If you want to automatically crop it to 18 char , then you have to follow the string slicing. But on you case its related field , so might be automatically cropped will not work.
Hope it will help you.