How to I stop a return parameter from a stored procedure from being truncated at 4000 characters

Question

The question asked here:-

How long does Access keep unflushed transactions locally?

is about problems with some invoice generation, which is still causing problems. I am now trying to rewrite the same code to work entirely on the backend SQL Server databases using ADODB, specifically ADODB.Command and a some stored procedures.

One problem I am trying to resolve is that in the old system I created a local table (called tmpUKRepeatInvoices) in the client. I am making a similar table on the server. However the fields of this table are created from a join across two separate databases.

It is easier with my new system to populate the equivalent table on the server associated with the invoicing with a stored procedure and using a FOR XML PATH('') clause in that stored procedure return a comma separated list of customerIDs (the keys to the other database) using a query like so

SELECT @Subs = (SELECT ''''+SubsID+''',' FROM InvoicingData WHERE SessionID = @SessionID FOR XML PATH(''),TYPE).value('.','NVARCHAR(MAX)')
SELECT @Customers = LEFT(@Subs,LEN(@Subs)-1)

@Customers is an output parameter for this stored procedure of type NVARCHAR(MAX) and will contain the text I will use in the next query

I can run the stored procedure in Sql Server Management studio and It returns a string of 42000 characters

The next query then will be like so. and can be used to populate the working table with the number of items against each customer.

SQL = "SELECT COUNT(*) As NoItems, CustomerID FROM CustomerItems WHERE CustomerID IN(" & Customers & " )"

So I create the call in my Access VBA to run the stored procedure like so

    Dim Customers As String
    With cmd
        Set .ActiveConnection = Conn
        .CommandType = adCmdStoredProc
        .CommandText = "CreateInvoicingData"
        .Parameters.Append .CreateParameter("@SessionID", adVarWChar, adParamInput, 25, TempVars!SessionID)
        .Parameters.Append .CreateParameter("@InvoiceDate", adDate, adParamInput, , Form_Company.InvoiceDate)
        .Parameters.Append .CreateParameter("@Currency", adVarWChar, adParamInput, 15, "Pounds")
        .Parameters.Append .CreateParameter("@Customers", adVarWChar, adParamOutput, 1000000000)
        .Execute
        Customers = .Parameters("@FleetCustomers").Value
    End With

The problem that I am having is that the Customers string is truncated to 4000 characters. I tried setting the @Customers parameter to type adLongVarWChar instead but an error was thrown at the .Execute statement that said that "Data type Ox63 is a depreciated large object or LOB, but is marked as an Output parameter. Depreciated Large Objects are not supported as output parameters. Use Current Large Objects instead."

What type should I use for the @Customers Parameter so it won't get truncated or regarded as depreciated?. Note: I tried setting the size of this parameter to -1 but that just failed with an inconsistent parameter error message.