You can find our current list of available Microsoft Online Services Powershell cmdlets @
http://technet.microsoft.com/en-us/library/cc742577.aspxIn addition, we are about to release (3) more on our next release:
http://blogs.technet.com/msonline/archive/2009/10/14/service-update-october-2009-release-availability-features-announced.aspxTo answer your question, we currently do not offer a powershell cmdlet to increase a users mailbox quota. Please open a service request within your Microsoft Online Administration Center to submit a feature request.
Thank you
-William Fiddes