You cannot. The BeOS for Intel Architecture includes a special edition of Partition Magic to make partitioning your hard drive and installing fairly painless, but the version included is a "lite" version of Partition Magic. It does not have all of the features of the full commercial version of Partition Magic.
The full version of Partition Magic allows you to choose any partition size you want, resize partitions quickly and easily, and includes additional (improved) features as well. If you want more functionality than the bundled version gives you, you should contact PowerQuest to see about upgrade pricing: