With previous version 1.0.0, expanding the SD card was a manual operation. However with the introduction of KiOS this is done automatically, the first time the system boots.
If you have installed Kerberos.io on Raspbian, you can just use the raspi-config command.
Procedure for version 1.0.0:
To install kerberos.io, you will need at least a 4GB or bigger SD card. By default the root partition of kerberos is 3.2 GB, this means that if your SD card is 4GB, 0.8 GB of your SD card isn't used. This is also true, if you're using a 8GB SD card, but then 4.8 GB isn't used.
If you want to use the complete storage of your SD card, you will need to resize your root partition. You can follow the expand SD card procedure on the FAQ page; be aware that according the model of your Raspberry Pi, another procedure is applied.