How big a partition for SuperDuper?

I have a 500GB internal drive on my late 2009 Mac mini and a 2TB external hard drive. I plan to clone the mini's HD to make a bootable backup (ie., backup all files on the internal HD). The partition size on the external hard drive should be 500GB, correct? Does it need to be a bit bigger for breathing room for the Smart Update feature? If so, how much bigger?

[dnanian]

It really depends on the kind of changes you made, how large the data is, etc. The worst case is a union of the old drive and the changes made on a temporary basis (e.g. moving a huge folder of photos where the new location is encountered before pruning the old one).

Of course, if this is a rare occurrence, it's easily cleared up with an erase-then-copy (or by helping Smart Update by moving/renaming the same folder on the backup).