Can you please be a good lad and describe what you are trying to do ? There is no built in size restriction, but you can't fit more than an Ethernet payload on an Ethernet frame, you know that.
TCP is stream oriented, so you can send whatever you want to send and will flow down the stream in segments. You just can't tell TCP to buffer more than what its buffer can buffer. See lwipopts.h
UDP is message oriented, so you have to split your data in datagrams, because a datagram must fit on an IP datagram on an Ethernet frame.