BGP for disaster recovery site

[CHRISTINE.M.BERNS at sargentlundy.com (CHRISTINE.M.BERNS at sargentlundy.com)]

We currently have a routable block (class B) of IP addresses.  We are in 
the process of designing a disaster recovery site.  Our main site is 
already dual homed to two different Internet service providers via BGP.  A 
consultant told us that in order to allow us to test access to the DR site 
without affecting the production environment, we should get another block 
of addresses from ARIN and advertise those addresses out the DR site's 
Internet connection.  Can we even expect to get another block from ARIN if 
we already have a class B, and could we not accomplish the same thing by 
advertising a subnet of our existing Class B at the DR site?  I would 
actually prefer to advertise a subnet of our class B,  but am wondering if 
there are any reasons why this is not a good idea.  Also, I have seen 
reference to some Internet service providers possibly not accepting /24 
BGP routes and either dropping them or aggregating them to  a  /21 or /20 
or /19.    Are there recommendations as to what  is the longest prefix 
that we should advertise to guarantee that the prefix will be advertised 
throughout the Internet?

Chris