I tend to use 10Gb as a base for my system drives, I just make sure that my clients remember to clean the disk of installed hotfixes 
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Kind Regards
Tom,
Tom Howarth VCP / VCAP / vExpert
VMware Communities User Moderator
Blog: http://www.planetvm.net
Contributing author on VMware vSphere and Virtual Infrastructure Security: Securing ESX and the Virtual Environment
Contributing author on VCP VMware Certified Professional on VSphere 4 Study Guide: Exam VCP-410
