Duncan Epping has released a great post on how to partition your ESX 4.0 (vSphere) using scripting and the grafical installer

http://www.yellow-bricks.com/2009/05/27/partitioning-your-esx-host-part-ii/

You cannot define the sizes of the /boot, vmkcore, and /vmfs partitions when you use the graphical or text installation modes. You can define these partition sizes when you do a scripted installation.

The ESX boot disk requires 1.25GB of free space and includes the /boot and vmkcore partitions. The /boot partition alone requires 1100MB.

The vmcore is created automatically by the installer

Primary:
/     - 5120MB
Swap  - 1600MB
Extended Partition:
/var  - 4096MB
/home - 2048MB
/opt  - 2048MB
/tmp  - 2048MB