How do I determine a TCP segment’s length

The TCP payload size is calculated by taking the "Total Length" from the IP header (ip.len) and then substract the "IP header length" (ip.hdr_len) and the "TCP header length" (tcp.hdr_len).

The "Bytes in Flight" field shows the amount of data that has been sent, but not yet ACKed (seen from the perspective of the point of capture).

You can add columns by right-clicking the fields in the Packet Details pane and select "Apply as Column" from the context menu:
tcp.len
tcp.hdr_len
tcp.analysis.bytes_in_flight

Here you can read more about adding and customizing columns.

tcp.len https://www.wireshark.org/docs/dfref/t/tcp.html