Earth diameters from the scale factor and real sizes by providing them with the size table with only the real sizes column filled in and the others blank. Make sure that the student understand that the objects need to be close to, but not exactly, the correct size to minimize frustration. If students have difficulty selecting the objects to use, have students line up their planet cards in order from the smallest to the largest to aid in object selection. For older students or if you have sufficient classroom time, consider adding the Moon to the card for the Earth. Attach the object you have selected for each planet to an index card.
Transparent tape placed over the object works well to attach an object to the index card.
Compare the objects you have selected for the Sun and the planets to the object you have selected for the Earth. This can be used as a follow-up to the scaled sizes portion of the activity. The scale of the solar system is immense. You can have your student groups make parallel model solar systems or have them radiate out from a central point. If at all possible, have extra adult supervision such as parents.