You can find out the size of a SP site following these steps
- using SharePoint designer  – on the root site information page  
 
- if you have service pack 1 you can view storage state using Storage Metircs : Site Actions > Site Settings > Site Collection Administration > Storage Metrics
- power shell :
 $site = Get-SPSite http://my.sitecollection.com
 $site.Usage;
 $site.Dispose();
