git-svnimport will not pack incrementally as it runs, so it might get pretty large. git-svn offers and incremental repack every x commits (I chose 1000) and that did wonders for the import time for me. Otherwise it will create a huge number of files before the final pack.
FYI, doing a repack brought the size down from 5.0GB to 266MB. Aside from the question of whether or not it's wise to require users to manually pack their repositories, it looks like the git tree is going to be about 750MB, which brings it more or less on par with mercurial. I'm keen to play around with mercurial a bit. Danny, when is your repository going to be publicly available? Ollie