If I want to know the number of bytes that I sent from my Gmail to email@example.com, can you help me? I can match all messages that I sent to firstname.lastname@example.org but from what I see it won't display data size.
That's not something you can do from within Gmail. At best, you'll need to work out a way to export all of the messages to another location and use a different tool to sum up their sizes. Even then, that's only going to give you an approximation.