To understand the size of site collection, you can use DiskUsed property in PowerShell console.  For example:


If you use SharePoint 2010, you need to add before this code command "Start-SPAssignment  -Global" otherwise it returns empty value. Don’t forget to add "Stop-SPAssignment -Global" after code.


To determine size of site collection in SharePoint 2010 you can use another property called "Usage.Storage". This property doesn’t require "Start-SPAssignment  -Global"