So, how big are the planets if the Sun is 18 inches across?

Jupiter, the biggest planet, would be almost 2 inches in diameter.

Pluto, the smallest planet, would be about the size of the period at the end of this sentence.

Here are the rest of the planets; their scale sizes and orbits :
To calculate the planet sizes and orbits for our scale model, we used the handy Solar System Calculator on the Exploratorium web site. You can too! Go to http://www.exploratorium.org/ronh/solar_system/index.html