To answer your question directly, you can use drives up to two terabytes. For A/UX, there are practical limits to the numbers and sizes of the partitions which make anything more than a few dozen gigabytes unnecessary, but the point is you can use just about any size drive if you're willing to not care about wasted space.

As far as the boot partition is concerned, I saw this in the A/UX FAQ:

Take care that you don't use too large a disk, however. The fsck in A/UX Startup that checks the root filesystem cannot scan a partition larger than 2GB. A/UX's own fsck can check larger partitions just fine.

Click to expand...

That is available here:

http://christtrek.dyndns.org:8000/doc/aux/faq.html