I’ve seen this question passing by a couple of times now and I found myself going through the document to dig up the reference. I thought it would come in handy to document it:

Q) What is the disk size requirements for ESXi installable?
A) The minimal required disk size is 5GB (Page 24, ESXi Installable and vCenter Server Setup Guide), recommend is 6GB (KB: http://kb.vmware.com/kb/1026500)