Maximum segment size
The maximum segment size (MSS) is a parameter of the TCP protocol that specifies the largest amount of data, specified in octets, that a computer or communications device can receive in a single TCP segment, and therefore in a single IP datagram. It does not count the TCP header or the IP header. The IP datagram containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several fragmented pieces; either way, the MSS limit applies to the total amount of data contained within the final reconstructed TCP segment.
Therefore: Headers + MSS ≤ MTU
The Maximum segment size to avoid fragmentation = Largest datagram size that any host is required to be able to reassemble - IP header size - TCP header size
So most IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576 - 20 - 20)
and most IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280 - 40 - 20).
A lower MSS will ensure that fragmentation will never be done along the path but, in counterpart, the data/header ratio will be lower.
Further reading 
- Comer, Douglas E. (2006). Internetworking with TCP/IP. Vol. 1 (5/E ed.). Upper Saddle River, NJ, USA: Prentice Hall.
- Kozierok, Charles M. (2005-09-20). The TCP/IP Guide (3.0 ed.). Retrieved 2011-09-08.