Here you need to choose a size for the new partition. This will become the size of the C drive, the main drive on your PC that Windows XP will install to. This is also the drive that all of your software and data will probably reside on unless you have additional partitions set aside for those purposes.
Unless you are planning on creating additional partitions from within Windows XP after the installation process (for any number of reasons), it's usually wise to create a partition at the maximum size possible.
For most users, the default number provided will be the maximum space available and the best choice. Press Enter to confirm the partition size.