There is the
lsof terminal command. This has a size field (7th field by default). This has a plethora of switches. Can probably tell you all you need to know about data connections.
sudo lsof -i : this will give you a complete list of all open network connections. Does not appear to give you the amount of data written or read from each socket though.
When searching for well-known ports, such as
8080 which is listed as
http-alt due to its mapping in
/etc/services, it might be easier to use
lsof -i -P to suppress conversion of port numbers to port names. Alternatively, explicitly specify the port one's looking for, like
lsof -i tcp:8080. When just trying to figure out what services are actually listening, use
lsof -i -P | grep LISTEN.