I'm never using that much per mbox - mostly posts from text-only
newsgroups, and seldom more than 40,000 posts per folder, since
selecting all the posts in a folder gets very slow (hours, if not days)
if the folder contains more than about 40,000 posts.
I'm seeing signs that I may need to disagree with something in that
article - the part about no limit on the number of folders. My
observations indicate that there is a number of folders above which
emptying deleted folders folders from Trash gets very slow - enough
that shutting down Thunderbird, then restarting it with the folders
emptied from Trash, is actually faster than waiting for the Trash
to be emptied normally. The number of odd problems increases as the
number of folders (counting all levels of the folder hierarchy)
increases. For example, many folder suddenly lose their message
counts, and with an even higher number of folders, some folder
start suddenly losing their contents. The number of folders at
which this occurs is too many to count - probably a few thousand.
My last two newsreaders (Windows Mail and Windows Live Mail) had
enough problems with a 4G limit on file size that I'm now watching
for very large files, and trying to avoid them unless they are used
only by programs running in 64-bit mode.
One of my computers (64-bit Windows Vista) is showing problems that
appear to be related mainly to the number of Tb folders. It's not
yet showing problems with the total numbers of files, even though
it now has about 7,300,000 files, mostly news posts, but not all of them
imported into Tb yet so that I not find and delete any duplicates.
Deleting duplicate message is currently enough to keep enough free
space on the C: drive.
The other computer (64-bit Windows 7) appears to be running low on
free space on the C: partition (which is on an SSD drive and therefore
fast but not especially large). It also has two nearly empty hard
drives that I could move the Tb files onto if I had more information
on how to do that. It's currently running a very long program that
will lose months of its work if interrupted to install anything that
requires other programs to be shut down during the installation.
It's also in use for most of my daily Tb use (on a different CPU
core).