100T seems to work for light use.
I can create the filesystem, mount it, write files and directories and read them back, and fsck doesn't report any problems.
Filesystem Size Used Avail Use% Mounted on /dev/vda1 99T 129M 94T 1% /sysroot
Linux (none) 3.1.0-0.rc6.git0.3.fc16.x86_64 #1 SMP Fri Sep 16 12:26:22 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux
e2fsprogs-1.42-0.3.WIP.0925.fc17.x86_64
qcow2 is very usable as a method for testing at this size.
Rich.