Textbooks are notorious for completely out-of-whack drawings of the planets in our solar system. They're about as out-of-whack as the solar system models made by 3rd graders - you know the ones, made from styrofoam balls. The craft stores only sell about 3 sizes of styrofoam balls, so Jupiter ends about being about twice as big as Mercury and Pluto. And the sun is maybe a little bigger than Jupiter...
Remember these pictures? They're a good place to start your discussion of planet size.
But, why not add some props to make the lesson even more fun and memorable.
For each planet (and the sun) I'll give you the object I used, the planet's actual diameter and the scaled diameter (so you can take your ruler with you to the produce department in search of the perfect melon to represent Saturn).
1,400,000 km --> 140 cm
1/2 plastic tablecloth
4,900 km --> 0.49 cm
12,100 km --> 1.21 cm
(in the pictures, I used a Gobstopper for both Venus and Earth because my marshmallow was missing)
12,800 km --> 1.28 cm
6,800 km --> 0.68 cm
143,000 km --> 14.3 cm
small (size 3) soccer ball
120,000 km --> 12.0 cm
51,800 km --> 5.18 cm
49,500 km --> 4.95 cm
2,300 km --> 0.23 cm
I like to begin the demonstration by showing students the sun and then having them guess how big Earth would be at that scale; everyone holds up their hands to show me how big it would be. They always think it's way bigger than it is!
After revealing the Earth, we then go back to Mercury and work our way through all the planets in the same way.
It's a good way to begin a discussion of Pluto's classification, as Pluto looks absolutely puny after those gas giants.