The measure

Hold your pencil at arm's length, upright, with your elbow straight. Line its tip up with the top of the kettle and slide your thumb down to the bottom. That is one "kettle".

Now turn the pencil sideways without bending your arm: how many of those is the kettle wide? Use the same pencil to check where the spout starts, or how big the handle is.

You are comparing one part of what you see with another part. Size on the paper is up to you; the proportions come from looking.