set transaction use rollback segment xxx

When using this statement; SET TRANSACTION USE ROLLBACK SEGMENT foo, does that transaction wait for the segment if it is in use?

We have 4 small rollback segments (2MB each), and 1 large rollback segment(for batch at 230MB). I want to set the batch queries to use the large rollback segment. BUT, i can't prevent the other users from using it either. So what happens if a user grabs the first extent of my large rollback segment, then my batch job tries to use it? Does the batch job grab the next extent (which is considerably smaller), then run out of space? Or does it wait for the initial extent? How are these segments and extents shared and used? Should I break up my 230MB segment into smaller chunks?

any help would be appreciated.
thanks,

-- 
Glenn Travis
DBA
Circuit City Stores, Inc.
Richmond, VA  USA
gtravis_at_richmond.infi.net
gtravis_at_aol.com