You might be surprised to learn that despite the registry's importance, its size is limited. The most important parameter that the Windows OS uses to calculate the registry's maximum size isn't disk space but rather available memory. The Windows OS uses this parameter because the OS keeps most of the registry data in memory during a session. By default, the registry's maximum size is set to a fraction of the paged pool size. The paged pool is the memory that the system can swap to disk; all user memory is part of the paged pool.
In Windows 9x, you can't change the value that specifies the registry's maximum size. In Windows 2000 and Windows NT, you can change this value, but in no case can the maximum size be less than 4MB or more than 80 percent of the available memory. The maximum amount of registry space is only an upper limit. In other words, setting this value doesn't cause space allocation or actual use of that space. For information about how to set this value, see the Microsoft articles "Understanding and Configuring Registry Size Limit (RSL)" (http://support.microsoft.com/support/kb/articles/q124/5/94.asp) and "INFO: Global Quota for Registry Data" (http://support.microsoft.com/support/kb/articles/q94/9/93.asp).