Quick Tip -- To get the storage usage of each site collection in a particular web application, use the following command in PowerShell:
Get-SPWebApplication http://webapplicationurl | Get-SPSite -Limit all | select url, @{label="Size in MB";Expression={$_.usage.storage/1MB}} | Sort-Object -Descending -Property "Size in MB" | Format-Table –AutoSize
Have a good day!
