How to predict the growth of a PostgreSQL database
Well, for this part of your question:


  I'm looking for a way to predict growth of the database given its fields, indexes,  constraints (as foreign key), number of lines, and so.
  
  I searched on Google, but I found only means to determine the actual space occupied by the database, and can not predict their growth.


It's not clear why monitoring pg_database_size(your_database_name) doesn't give you the information you are after. Plot the size of all your databases daily, fit a curve to the plot, and you should be able to come up with an estimate of when you need to buy more disk space. What else do you need to know, and why?

For this part of your question:


  For example, how can I determine how many bytes would be occupied on disk for the following tables, in the following cases
  
  1) 100,000 rows in the account table, 10 rows in the role table and 100,000 lines in account_role table.
  
  2) 500,000 lines per account table, 20 lines on paper table and 600,000 rows in the table account_role?


You don't really need to concern yourself with how many rows are in each table or the precise structure of the table. Just query SELECT pg_total_relation_size('account'::regclass); and you have the answer, in bytes.