Error - Output Param - SP - Size Property Has An Invalid Size Of 0

Using C#, SQL Server 2005, ASP.NET 2, in a web app, I've tried removing the size from parameters of type NCHAR, NVARCHAR, and VARCHAR.  I'd rather just send a string and let the size of the parameter in the SP truncate any extra chars if need be. 
I began getting the error below, and eventually realized it happened only with output parameters, as in the code snippet below.

String[3]: the Size property has an invalid size of 0.

  par = new SqlParameter("@BusinessEntity", SqlDbType.NVarChar);
  par.Direction = ParameterDirection.Output;
  cmd.Parameters.Add(par);

  cmd.ExecuteNonQuery();
What's the logic behind this? Is there any way around it other than either finding out what the size should be, or assigning a size larger than would ever be needed?
 
Thanks
Mike Thomas