Can someone please explain to me or atleast point me in the right direction so I can read about properly sizing a tablespace.

I have read several books,papers,help questions, etc on tablespace sizing and so far everything explains how Oracle allocates initial/next extents, etc. What I need to know is how do I calculate or guesstimate the proper setting for initial/next extent on the tablespace? For example. Let's say I have 20 tables varying in row size. I want to assign the tables into 2 tablespaces. How do I figure out what the best initial/next extent sizes need to be for that tablespace given the tables that will go into that tablespace?. I know what my db_block_size is. I know what the OS block size it.

Also, I have read that the best thing to do is to specify the extent sizes at the tablespace level and not at the table level. If this is the case, what happens to tables that are created with extent sizing options and the tablespace the tables were placed in was also created with the extent sizing options? Which sizing options are used?

Thanks.