Hi, I agree with John.
Most of the time I see people factor in about 30% for free space on each LUN during LUN sizing (15% for snapshot + 15% for free space).
When sizing a LUN for a VMware datastore, you would want to determine first how many active virtual machines to allocate per LUN.
For read/write performance, it does not matter if your VMDK is on a 100GB LUN or 2TB LUN.
It is the total I/O load on the LUN that matters.
If you'd like to design your datastore/LUN, how big should be the LUN, how many VMs per VMFS, etc, you can also use sizing tools created by Samir Roshan here - VMware VMFS Datastore Sizing Calculator: http://thinkingloudoncloud.com/calculator/vmware-vmfs-datastore-sizing-calculator/
Below are some links re: VMware Storage/VMFS best practices
VMware Storage Best Practices: http://www.vmware.com/files/pdf/support/landing_pages/Virtual-Support-Day-Storage-Best-Practices-June-2012.pdf
VMFS Best Practices: http://www.vmware.com/pdf/vmfs-best-practices-wp.pdf
Thanks,
Bayu