In unmanaged C++ and Win32, first open the serial port with CreateFile, then
call SetupComm.
In C++/CLI, instantiate a System::IO:orts::SerialPort object and set the
ReadBufferSize and/or WriteBufferSize properties.
Note that in either case, Windows and/or the serial port driver is free to
decide on the appropriate buffer sizes. Your settings are treated as
recommendations, not mandates.