UCC images may be to small for some needs. They may be grown by building a complete own image (like described in Generation of adapted UCC images) but this will need some time and effort.
The existing image (with ext3 or ext4 filesystem) can simply be grown to the desired size by using the tools truncate and resize2fs. To grow the image (usually found in /var/lib/univention-client-boot/) by one gigabyte, the following commands may be used:
truncate -s +1G <image file name>
fsck.ext4 -f <image file name> # fsck.ext3 respectively
resize2fs <image file name>