There is no way to increase it, but there are work-arounds you can read about in <a href=”http://support.microsoft.com/kb/886616″>this article</a>. You do not say whether you are connecting to an exchange server or not, but Exchange 2007 doubled the space allowed for rules. If this is not an exchange account try the workarounds.
for exchagne 2007 we can increase the rules quota using shel command
Set-Mailbox -id TEST-MBSA1SG01DB1 -RulesQuota 128KB