ESXi 4.0 resize a VM hard drive size

You can use the VMWare Converter to clone the machine, but change the size of the virtual disk in the process. When configuring the Source Data, indicate that you want to "Select volumes and resize to save or add space". You will then be able to specify the size of the destination drive in the "New Disk Space" column below. If you're simply trying to resize rather than clone and resize at this point, you can then remove the original once you verify the converted version works.

You can expand virtual hard disks, but you can't shrink them natively in ESX(i).

One popular strategy is to attach a second smaller virtual HD to the VM, run a cloning tool (Ghost, Acronis...) in the VM to clone the first disk to the second one and then replace the first disk with it.