Many thanks, this is what I looked for.
The only thing I don't understand is (sorry, I'm totally new to Spriter): why do you create a box_000, what is it? Can't you simply set the longsword as object?
And so, if I wish to know the bounding box of the entire sprite (or a significant portion of it, like in the image below) should I create a little sprite and adapt it to the wanted size?