If you are on a linux/unix machine, you could just use tcpdump. Something like:

sudo tcpdump host 1.1.1.1
[download]

will print a line for each packet sent to or received from 1.1.1.1. Most packet types will include the length at the end of the line, following the word "length" or in parentheses.

tcpdump is a pretty complete tool for working with libpcap. It is worth a look to avoid re-inventing the wheel.