128 Gb is overkill. Its more than enough. You can hit up to 35-40 bots with it or even more depends on how much RAM you give to your VM's. Considering just SSD space.
All you need:
Win7 VM = 8Gb Max
Diablo 3 on host pc shared over vm-ware network = 8 Gb (the network speed in this case is equal to your SSD speed as its inside one pc, no matter it says its only 1gbps)
So you create a VM and each next created VM is a linked clone from frist one which takes about 200mb + extra temporary boost in XX GB when run which depends on how much ram you give it (lets say 1Gb for Diablo is enough) so its extra 1,2 Gb
As result you need about 16 Gb HDD space + 1,2 * 4 Gb (1 bot on "big" VM lets say it is a template and 4 VM on copies) and you get shitload of space left.
Running 4 bots for now on 128 Gb SSD and have 80 gb left. (39 Gb is VMware + Diablo + Host os + cloned VM's and some work stuff)
How-To?
VMWare Linked-Clone (found in manual)
Shared folders in VM (as network drivers in guest os) (same)
mklink.exe (you can create Diablo folder in guest os and make links for all folders/files to network drive, except cache directory (it unfortunately requires to be independent)
So its easy to do.