For a full installation, IBM suggests that you create a Linux partition called /csminstall that consists of 1.5 GB of space for a full installation, which includes the operating system, or 100MB for a CSM-only installation. Note that for full installation, the 1.5 GB of space required is per Linux operating system -- if you have both Red Hat Linux 7.1 and 7.2, double this amount. The /csminstall partition will hold the required RPM packages for installation.
For a CSM-only installation, it is not necessary to create a separate partition. You can simply put the CSM software and files in the root partition.
If you are using Red Hat Linux 7.2, you can use fdisk to create the partition. If you are using Red Hat Linux 7.1, you can use either fdisk or cfdisk to create the partition.