Determining disk storage space required for a new installation
Before you schedule a time to install the new IBM® i release, you need to know the storage space that is required.
Load-source disk unit requirements: 
IBM i
7.2
Licensed Internal Code requires that you have
a load-source disk unit of 70 GB or larger
for each system or logical partition.
Options for upgrading the load-source disk unit are available under the topic Disk management checklist . Otherwise contact your authorized service provider for assistance.