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