The answer depends on factors you have not given.
If you are using Dictionary Managed Tablespaces, then the next extent will be the most current extent plus a percentage increase defined by the segment's PCTINCREASE storage parameter.
If you are using Locally Managed Tablespaces (LMT), then Oracle will automatically calculate your next extent size for you. I highly recommend using LMTs and quit worrying about extent sizes!
This was first published in May 2005