The following code has cost me allot of time. It was very hard to find the pieces and put them together. So I hope I can help a few of you with that post ;-)

The Problem

The problem was that I need to access different Exchange accounts via Exchange Web Services (EWS) and read out the mailbox size.

First

To do this I first used a view only account in the active directory that I have activated for impersonation by typing the following code into the Exchange-Shell:

Get-ExchangeServer
| where {$_.IsClientAccessServer -eq $TRUE}
| ForEach-Object {Add-ADPermission -Identity $_.distinguishedname -User (Get-User -Identity User1 | select-object)
    .identity -extendedRight ms-Exch-EPI-Impersonation}

You can find an explanation to this code on MSDN.

Second

Then I set up the connection string to the EWS:

// Certification Validation always true
ServicePointManager.ServerCertificateValidationCallback =
                    delegate(
                        Object obj,
                        X509Certificate certificate,
                        X509Chain chain,
                        SslPolicyErrors errors)
                    {
                        return true;
                    };

// Setup connection string
ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2007_SP1);
service.Credentials = new NetworkCredential("viewadmin", "password", "FQDN");
service.AutodiscoverUrl("viewadmin@FQDN");

Third

Thirdly I used impersonation to connect to another mailbox account:

// Impersonate
service.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.SmtpAddress, "user@FQDN");

Finally

And finally I requested the folder size with this nice little piece of code:

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

/// <summary>
/// Gets the size of the mailbox in kilobytes.
/// </summary>
/// <param name="service">The ExchangeService object.</param>
/// <returns>Returns the used kilobytes in double.</returns>
public static double GetMailboxSize(ExchangeService service)
{
    var offset = 0;
    const int pagesize = 12;
    long size = 0;

    FindFoldersResults folders;
    do
    {
        folders = service.FindFolders(WellKnownFolderName.MsgFolderRoot,
                                      new FolderView(pagesize, offset, OffsetBasePoint.Beginning)
                                      {
                                          Traversal = FolderTraversal.Deep,
                                          PropertySet =
                                              new PropertySet(BasePropertySet.IdOnly, PidTagMessageSizeExtended,
                                                              FolderSchema.DisplayName)
                                      });

        foreach (var folder in folders)
        {
            long folderSize;
            if (folder.TryGetProperty(PidTagMessageSizeExtended, out folderSize))
                size += folderSize;
        }
        offset += pagesize;
    } while (folders.MoreAvailable);

    return size;
}

I hope I could help you. If you have improvements on the code don´t hazle to comment ;-)