string siteCollectionURL = “http://localhost:80/sites/TestSite”;

SPSiteAdministration oSPSiteAdministration = new SPSiteAdministration(siteCollectionURL);

float scDiskUsage = oSPSiteAdministration.DiskUsed;

float sizeOfSiteCollection = (scDiskUsage / (1024 * 1024));  // In MB

string siteCollectionURL = “http://localhost:80/sites/TestSite”;

SPSite oSPSite = new SPSite(siteCollectionURL);

SPWeb oSPWeb = oSPSite.RootWeb;

float totalSubSitesUsage = GetWebSize(oSPWeb);

float GetWebSize(SPWeb web)

{

  float total = 0;

  foreach (SPFolder folder in web.Folders)

  {

    total += GetFolderSize(folder);

  }

  foreach (SPWeb subweb in web.Webs)

  {

    total += GetWebSize(subweb);

    subweb.Dispose();

  }

  return (total/(1024*1024));

}

float GetFolderSize(SPFolder folder)

{

 float folderSize = 0;

 foreach (SPFile file in folder.Files)

 {

   folderSize += file.Length;

 }

 foreach (SPFolder subfolder in folder.SubFolders)

 {

   folderSize += GetFolderSize(subfolder);

 }

 return folderSize;

}