In wireshark, you have this field:
[TCP Segment Len: 1380] # How is this calculated?
My assumption is using the IP headers plus TCP's.
TCP Segment Len = IP Total Length - IHL - TCP Header Length
In the packet trace, the values filled in would look like
[TCP Segment Len: 1380] = 1420 - 20 (5 bytes) - 20 (5 bytes)