It really depends on how fast your table is growing.
If your table is growing really fast, it wouldn't be unreasonable to make the next extent double the size of the last extent. After a couple of days, see how many times you have extended and re-evaluate.
To avoid fragmentation nex extent should be equal to initial extent.
For data tablespace in a DB with not very big table an atent of 128 k is what Oracle raccomanded.
However you should analyze with particular attention your DB structure and size
This month's Oracle Magazine has a very interesting article regarding tablespace management -Locally managed tablespace ~.
you can also see this issue online [url]http://www.oracle.com/oramag/oracle/00-nov/o60o8i.html[/url]