I've found that about 2 gigs is sufficient for my OS 9 stuff. The way I've got it is that anything that runs in classic goes on that 2 gig partition. ALl my other stuff goes on my X drive. I like the partition because it makes it ultra-easy to move stuff off the X drive in case something gets screwy and wipe the X drive clean (only happened once). So, what I would do is calculate the amount of space that your classic apps need and add a couple hundred megs for your system folder and that should be a good enough amount.
I do it this way so that I can eventually eradicated classic quickly and easily from my system (literally and figuratively). Still waiting for 2 apps: SPSS and dreamweaver. Who knows how long that will take
I forgot to add: With the 2 partitions, I don't have things messing around in the root level of my OS X drive. I like to keep my root clean. I see that as a big advantage of the separate partitions.
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I do it this way so that I can eventually eradicated classic quickly and easily from my system (literally and figuratively). Still waiting for 2 apps: SPSS and dreamweaver. Who knows how long that will take