What is the capacity of a table in PostgreSQL?

Maximum Table Size is 32 TB. You can use the following methods to get the size of table:

The pg_relation_size() function is used to get the size of a table. 
Syntax: select pg_relation_size('table_name');

To make the result readable, one can use the pg_size_pretty() function. The pg_size_pretty() function takes the result of another function and formats it using bytes, kB, MB, GB or TB as required. 
SELECT pg_size_pretty (pg_relation_size('country'));

32 TB