I believe you are referring to: ESXi Hardware Requirements when you mention the 5.2 GB disk space, however, please make a note of the following mentioned on the same page:
Due to the I/O sensitivity of USB and SD devices, the installer does not create a scratch partition on these devices. When installing or upgrading 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. After the installation or upgrade, you should reconfigure /scratch to use a persistent datastore. Although a 1GB USB or SD device suffices for a minimal installation, you should use a 4GB or larger device. The extra space is used for an expanded coredump partition on the USB/SD device. Use a high-quality USB flash drive of 16 GB or larger so that the extra flash cells can prolong the life of the boot media, but high-quality drives of 4 GB or larger are sufficient to hold the extended coredump partition. See Knowledge Base article http://kb.vmware.com/kb/2004784.
Please check the SD card compatibility with ESXi 6.5.
Also, proceed with taking a backup of all the VMs hosted via third party backup if possible. In the unlikely event that the upgrade fails and you do not have a way to revert back via altbootbank as well, you can proceed with a clean installation of esxi 6.5. Please refer the following kb to revert to a previous version of ESXi in case of a failed upgrade: VMware Knowledge Base