You can find out the size of a SP site following these steps

  1. using SharePoint designer  – on the root site information page site storage
  2. site storage2
  3. if you have service pack 1 you can view storage state using Storage Metircs : Site Actions > Site Settings > Site Collection Administration > Storage Metrics
  4. power shell :
    $site = Get-SPSite http://my.sitecollection.com
    $site.Usage;
    $site.Dispose();