There is no size limit, only time limit. If you want to transfer 1tb of data, you can. But itll take some time tho. If you are asking for buffer size, that is(as already said) determined by the software in the other end. If you are referring to TCP/IP packet length that would be 1500 bytes and UDP ~500 bytes(?). Tho all of those sizes is determined by the OS so I guess they can be increased somehow.