No different than booting locally. Min is around 10GB I believe, but you are better off with more like 20-30.
VCP, vExpert, Unix Geek
Even though I wouldn't recommend to place all eggs in one basket, if your SAN crashed, all your hosts and VMs are BSOD. If you have a need to boot from SAN, make sure to verify the storage array has enough "host group partitions" available and can read more details here http://vmetc.com/2007/09/26/esx-boot-from-san-design-limitations/
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VMware vExpert 2009
iGeek Systems Inc.
VMware, Citrix, Microsoft Consultant