Hi,
ESXi Hardware Requirements , please refer
Storage Requirements for ESXi6.5 Installation or Upgrade
Installing ESXi6.5 or upgrading to ESXi6.5 requires a boot device that is a minimum of 1 GB. When booting from a local disk, SAN or iSCSI LUN, a 5.2-GB disk is required to allow for the creation of the VMFS volume and a 4-GB scratch partition on the boot device. If a smaller disk or LUN is used, the installer attempts to allocate a scratch region on a separate local disk. If a local disk cannot be found the scratch partition, /scratch, is on the ESXi host ramdisk, linked to /tmp/scratch. You can reconfigure /scratch to use a separate disk or LUN. For best performance and memory optimization, do not leave /scratch on the ESXi host ramdisk.