Installing ESXi on a flash drive is as easy as attaching an USB or SD device with at least 1GB capacity (some server types have a built in SD slot on the main board), ensuring that it is in the boot options and then selecting this device while installing ESXi from the CD.
Anyway, I'm a little bit confused about the RAID/VD sizes you mentioned. They actually don't match with the disk size. Can you provide the details from the RAID configuration, how the disks are used?
André