I want to learn more about Table/Columns/Indexes design.
How big the columns should be?
How many number of columns should a table have at max?
How many rows a tables should have at max
What are size limitations of different data types
Any books, blogs , videos, courses that you recommend I go over?
I have been an Oracle DBA , but only from a support and tuning standpoint. I have never done this kind of work before, even in Oracle. So interested in learning in this area?
Any approach that will speed up my learning?
The Postgres docs, IMO, are astonishingly good. https://www.postgresql.org/docs/ -- Not what I tend to expect from open source projects, but there's a reason (again IMO) Postgres is the best database solution ever built. Regarding your specific question here are the current "hard" limits https://www.postgresql.org/docs/current/limits.html
But recognize that the effective limits on your system are going to be lower, and based on your available ram, disk, and other resources.
Stackoverflow might be a good place to understand more about these kinds of limits. I'll say that for my use cases, I've never had to worry about anything other than applying the right index or reworking my SQL statement to solve a particular problem in Postgres. So I'd say try to learn about various index techniques and how to use Postgres' SQL, as the starting place to learn. I think EnterpriseDB has good resources for those coming from Oracle as well..