USB packet size C#

Hello,

I am sending packets (string of bytes) from USB dongle to C# GUI.
But when I am sending more than 60 bytes my GUI stack.
I am receiving bytes in GUI using:
         System.Threading.Thread.Sleep(500);
            string Text = "";
            byte[] buff = new byte[255];
            //int tt ;
            port.Read(buff, 0, 255);
Is there any option to define maximum number of bytes that my GUI can get via USB device?
Where can be a problem?

THANKS