This post is a copy from Cody Bunch his blogpost. Cody thanks for sharing!!!:
This is a tool that I created a while back to assist in sizing needed disk space in a deployment. Straight forward to use, the totals are calculated as follows: VMDK Size + Ram Size * 1.1 + 12Gb = Total Needed. While the VMDK may be obvious, the others are just as important. Ram Size is included, as ESX will create a swap file on the disk where the VM’s configuration resides (unless you specify otherwise) and needs to be included. The * 1.1 is to add 10% to the overall solution, to allow for snapshots. This can likely be adjusted up or down depending on your specific requirements, but I’ve found that at least 10% works best. The last number, 12GB. This one may seem like a mystery, and likely it is. This was originally included to allow for the ESX install on a local host, and some additional room for it’s related overhead. If your VM’s are on SAN, this number can likely be left out, but… a little more overhead never hurt anyone.
Download link and source: http://professionalvmware.com/2008/12/08/virtual-machine-disk-sizing-tool/