To find out how much disk space you are using:

  1. Log in to CONTROL PANEL and go to Services > Mailboxes > Disk Quota Management.
    Disk Quota Management
  2. You will see a bar that indicates your disk space usage at the last update. This number is updated every 24 hours, at approximately midnight PDT.
    Disk Space Usage

This page also shows you how much disk space is included in your plan and how much additional disk space you have purchased. To increase your disk quota for your account if you are running out or over the limit, select the amount you want to purchase and click the Save Changes button.
Additional Disk Space

To manage your disk space per mailbox, you can go to Services > Mailboxes > Disk Quota Management section and select Manage Individual Mailboxes option on top of the page.
Manage Individual Mailboxes

Read the Knowledge Base article on How to use storage management on Exchange accounts for additional instructions on changing the size limit on each mailbox.