As was mentioned, HWU (http://hwu.netapp.com) is your friend, but here's a quick cheat-sheet from:
http://www.datacenterdan.com/blog/netapp-dataontap-83-adp-root-disk-slice-deep-dive
The root partition size is fixed and automatically set per controller. The actual amount will vary for the total of the root data partition between 430.9GiB (462.7GB) to 431.5GiB (463.3GB). The reason for this 0.13% fluctuation is due to the differing numbers of 4k blocks available with different spindle counts. Of course, the OCD in me wishes it was always just 450GB for a nice round number!
The way that root partition is parceled out is then based upon the number of disk drives with root partitions, calculating for the appropriate RAID level and hot spares as well. Here are the common disk configurations and their corresponding root disk sizes:
FAS2520 (12 Internal HDD) FAS2552/4 (24 Internal HDD) FAS2552/4 (20 Internal HDD) FAS8000 AFF (24 SSD) FAS8000 AFF (36 SSD) FAS8000 AFF (48 SSD) | 144GiB per root slice 54GiB per root slice 62GiB per root slice 54GiB per root slice 31GiB per root slice 22GiB per root slice |