C#

[DeploymentItem("Content/Importcontent/import")]

PS C:\Users\> gwmi -namespace "root" -class "__Namespace" | Select Name

Name
----
subscription
DEFAULT
QuestSoftware
MicrosoftDfs
CIMV2
Cli
Nap
MicrosoftIISv2
MicrosoftActiveDirectory
SECURITY
SmsDm
snmp
RSOP
ccm
WMI
IntelNCS2
directory
Policy
Hardware
ServiceModel
Microsoft
aspnet

 private static readonly ExtendedPropertyDefinition PidTagMessageSizeExtended
                        = new ExtendedPropertyDefinition(0xe08, MapiPropertyType.Long);

private void button1_Click(object sender, EventArgs e) // test
        {

            ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2007_SP1);

        
            //Change the Credentials to suit you needs
            service.Credentials = new WebCredentials("[account]", "[password]", "[domain]");
            

            //Use Autodiscover or Set the URL manually. Change the email address to match yours
            service.AutodiscoverUrl("emailaddress@domain.com");
            
            int moffset = 0;
            int mPageSize = 500;
            FolderView view = new FolderView(mPageSize, moffset, OffsetBasePoint.Beginning);

            // Identify the properties to return in the results set.
            view.PropertySet = new PropertySet(BasePropertySet.IdOnly);
            view.PropertySet.Add(FolderSchema.DisplayName);
            view.PropertySet.Add(FolderSchema.ChildFolderCount);
            view.PropertySet.Add(FolderSchema.TotalCount);
            view.PropertySet.Add(FolderSchema.UnreadCount);
            
            // Unlike FindItem searches, folder searches can be deep traversals.
            view.Traversal = FolderTraversal.Deep;//.Shallow;
            //FindFoldersResults folders = service.FindFolders(WellKnownFolderName.MsgFolderRoot, view);
            FindFoldersResults folders = service.FindFolders(WellKnownFolderName.MsgFolderRoot, view);


            var offset = 0;
            const int pagesize = 500;
            long size = 0;

            FindFoldersResults foldersNew;
            do
            {
                foldersNew = service.FindFolders(WellKnownFolderName.MsgFolderRoot,
                                              new FolderView(pagesize, offset, OffsetBasePoint.Beginning)
                                              {
                                                  Traversal = FolderTraversal.Deep,
                                                  PropertySet =
                                                      new PropertySet(BasePropertySet.IdOnly, PidTagMessageSizeExtended,
                                                                      FolderSchema.DisplayName)
                                              });
                foreach (var folderNew in foldersNew)
                {
                    object folderSize;
                    if (folderNew.TryGetProperty(PidTagMessageSizeExtended, out folderSize))
                    {
                        //Console.Out.WriteLine("{0}: {1:00.00} MB", folderNew.DisplayName, folderSize / 1048576);
                        int bytes = Convert.ToInt32(folderSize);
                        int megaBytes = (bytes / 1048576);
                        size += megaBytes;
                        MessageBox.Show(folderNew.DisplayName + ": " + megaBytes + ", total mailbox size: " + size);
                    }
                }
                offset += pagesize;
            } while (folders.MoreAvailable); 
}