i believe that value can be configured for a certain System Package Type.
Go to "Confiugration" -> "Provisioning Configuration".
In there open the System-Package-Type that you are using and set the value under "Initial Partition Size".
Then i'm afraid i also have no idea on how to do that!
15GB was set by design due to minimum requirement for Windows 2008+ 64bit.
However I found that even 15GB is barely enough to install SP1 on the system, provided that you turn off hibernation, do not use paging file, and point to SP installer that's on the remote drive (to conserve every bit of space)