> What is the partition limit of the fs-dos.so file system driver?

From the Users Guide (Understanding System Limits):

"
DOS FAT12/16/32 filesystem

Filesystem size
    Depends on the FAT format:
        * for FAT12, it's 4084 clusters (largest cluster is 32 KB, hence 128 MB)
        * for FAT16, it's 65524 clusters (thus 2 GB)
        * for FAT32, you get access to 268435444 clusters (which is 8 TB)

Disk size
    Limited by the disk driver and io-blk.
"

The 2G/8T assumes a cluster size of 32k, as Microsoft suggests avoiding 64k for maximum compatability (although I think 
fs-dos is sign-safe); FAT32 defaults to 4k.  The disk subsystem uses 32-bit blknos, so you'd hit that 2TB limit first on
 a HDD with 32k clusters; with more usual 4k clusters you'd hit the FAT32 fsys limit first.  Select appropriate minimum 
(although either way it is a fairly large number; this is theorectical, I've never had a HD larger than 250G).