Expanding on Gerald
's correct reply, due to intellectual property issues CentOS does not provide tools to manipulate NTFS filesystems. If you do not have available unpartitioned space then you can use a Linux LiveCD
that does provide such tools to shrink Windows partition[s] to free space for a CentOS install. Depending on the intended purpose for the system, and how many packages or groups you want to install, a minimum of 10 to 20 GB would be recommended - more is better to avoid a squeeze in the future.