Here's what I find to be a handy rule to help get a grasp of the relative sizes of the Earth and Sun and the distance between them. I'm sure I'm not the first to "discover" this but I haven't found it elsewhere.
The rule is, the Earth-Sun distance is about 100 times the diameter of the sun, and the diameter of the sun is about 100 times the diameter of the earth.
Since there is actually a unit for the distance between the Earth and the Sun, the AU, this means the Sun is about 0.01 AU and the Earth is about 0.0001 AU.
Another way of looking at it is if the Sun were only 1km away, it would be about 10 meters in diameter, and the Earth would be about 10 cm in diameter.
Yet another way of looking at it, for those whose brains are adjusted to finance, is that the Sun's diameter is about 1% of the Earth-Sun distance and the Earth's diameter is about a basis point of the Earth-Sun distance.
