To calculate the size of the hard drive needed to install and back up Salesforce, you need to know the storage usage values in your Salesforce account.
Sign in to Salesforce with an administrator account.
In the upper right of the page, click Setup.
In the navigation pane, go to Administrator > Data Management > Storage Usage.
The Storage Usage pane displays the Data Storage and File Storage space in the Used column. You must use both of these values to determine the space.