It is roughly the MTU minus the IP and TCP headers (20 + 20 bytes without options), so 1460 on a 1500 MTU. Each side announces its MSS when the connection opens.
MSS
Maximum Segment Size: the most TCP payload bytes per segment.
It is roughly the MTU minus the IP and TCP headers (20 + 20 bytes without options), so 1460 on a 1500 MTU. Each side announces its MSS when the connection opens.