This article outlines how to get the site quota details for all the sites under specific web applications and for all the sites under a SharePoint Farm using a PowerShell script.
Site QuotaA quota specifies the storage limit values for the maximum amount of data that can be stored in a site collection. Quotas also specify the storage size that, when reached, triggers an e-mail alert to the site collection administrator. Quotas can be saved as quota templates that can then be applied to any site collection in a SharePoint Farm. Using quota templates rather than individual quotas can simplify setting storage limits on new site collections.The storage limit applies to the site collection as a whole. In other words, the storage limit applies to the total size of the content for the top-level site and for all subsites within the site collection. If versioning is enabled, the versions in a site and the content in the Recycle Bins count toward storage limits. You can also specify a percentage of storage limits for the second-stage Recycle Bin.The script collects the following information:
SharePoint 2013 Administration Essentials