A little googling mentions a 200MB HD size. So it could be worth trying something smaller. Didn't older A590/2091 drivers fail with >1GB disks? Maybe there could be a similar bug in the AMIX code.
For partitioning, might you need to create a UNI\0 partition using HDToolBox first? (I don't know, is there a scanned version of the AMIX install guide online? I have a printed copy
somewhere which I can try to find if you get stuck.)
Oh, I just found
this. PDF manuals, HD image of installed AMIX OS, etc. etc.
Edit: See
this post for a link to a PDF of the Installing Amiga UNIX manual.