It depends. Messages sent across ethernet are usually described as Frames. It has two parts. First is the header containing information about the frame. The frame is the actual data being transmitted. The frames are in variable length depending on what is being sent.
What is it exactly you’re trying to do? Monitor network traffic? What is actually being sent? Need more info..
10BaseT is actually at the physical layer of the OSI model. It is the ethernet specification for unshielded twisted pair cable (category 3, 4, or 5). The 10 signifies 10 Mbits per second. Base indicates that the type of signaling used is baseband. The T at the end means that twisted-pair cable is used.
For most environments, the maximum frame size on 10BaseT is 1514 bytes – which includes all the header information mentioned above. Bear in mind that the header size changes depending on the protocol family, and the number of layers of protocols in a given frame or packet. Without going into exhaustive detail, I’d repeat LabNuke’s question – “What is it exactly you’re trying to do?”