In ADUC, select the user whom you want to increase their mailbox. Right click or double click their user account. go to the Exchange General tab and then click on the Storage limits button. If you unclick the box for “Use mailbox store defaults”, you can then set any mailbox limits you wish for the user.
It is not recommended that you use ADUC to modify Exchange 2007 accounts. Instead open the Exchange Management Console, go to the Recipient Configuration and click mailbox. You can modify the indiviual user settings by going to the properties of the user. OR you can move the user into another mail store with higher quotas which would be easier to manage on your side administratively but would require a downtime of approx an hour to the user. You could also modify the quota setting on the entire mail store if you think you have made it too constrictive. So you have several options. But definitely use Exchange Management Console whatever choice you decide is best.