F11 live CD install Q re dual boot partitioning

[Quinney, Matthew]

When installing F11 from the Live CD, during the partitioning for dual
boot, there is a box labeled "volume to shrink" with a drop down menu
for the volume and a size box labeled "shrink target(in MB)".  Is this
the size by which to shrink the original volume or is it the size to
which the volume will be shrunk.  It is kind of an important distinction
if you want to preserve what you originally had, but I have not been
able to figure that out from an afternoon of looking through docs and
past list questions (perhaps I'm not searching using the right
phrases?).
 
Thanks,  Matt