2 ГБ будет достаточно, только не забудьте потом сконфигурировать отдельно scratch partition.
Storage Requirements for ESXi 5.5 Installation
Installing ESXi 5.5 requires a boot device that is a minimum of 1GB in size.
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Due to the I/O sensitivity of USB and SD devices the installer does not create a scratch partition on these devices. As such, there is no tangible benefit to using large USB/SD devices as ESXi uses only the first 1GB. When installing on USB or SD devices, the installer attempts to allocate a scratch region on an available local disk or datastore. If no local disk or datastore is found, /scratch is placed on the ramdisk. You should reconfigure /scratch to use a persistent datastore following the installation.
Отсюда: vSphere 5.5 Documentation Center