You will need 2 USB drives - first 8-16GB size and second at least 32GB (bigger = better = more space for datastore).
Create ESXI installer on smaller one, stick it to server.
When installing ESXI (i'm using version 7 don't know if this will work with 6.x) do same as NetworkChuck did on his RaspberryPi:
So during boot edit command:
runweasel cdromBoot
to look like this:
runweasel cdromBoot autoPartitionOSDataSize=8192
This way ESXI partitions will be limited to 8GB (you can use 16GB = 16384 to be sure that future updates install without issues).
Install ESXI on bigger USB drive and after that you end up with datastore on same drive.