The current ATA specification for the maximum size of IDE drives is 137GB, but this is likely to change soon. The ADAT/FST file system could support drives up to 2.2TB (2200 GB), and a firmware upgrade would make larger sizes available to the HD24. Common limitations on hard drive sizes for FAT32, HFS, etc. are not applicable. So, in the future, as long as the drive physically fits in a 3.5" disk caddy, the disk space could be almost as many gigabytes as you want.
Having said that, consider that an 80 GB drive (available today for around $250) holds 6 hours of 24-track audio (that's 8,640 track minutes), the equivalent of 24 forty minute 8 Track ADAT tapes (actually more expensive at around $350).