There are two ways to check how much space was used on your SecurusVault account.
You can log into your web account, and click on your profile tab on the top right corner.  Then, click on the Devices tab on top of the page, and there will be a used space bar that tells you how much space you've used so far.

You could also check the amount of space used when you open your SecurusVault application, and at the bottom of the window, there is a Storage bar that displays the amount of space used on your SecurusVault account.