In little plastic sandwich bags, I have a collection of items. If you do this with several children, make sure each bag has the same items. I then give each child or group the Venn diagram below. (Note: You can click on the picture to get a full size render you can save and print!)
Then I announce which item they are going to compare the others to. In the example below, they're going to compare the other items to a penny.
The students then place the items in the correct areas. With Pre-Kinders, we make it a race to see who finishes first with all the correct answers (emphasize correct answers).
In Kinder, once they're done, the children then have to write down the names of the items (purple eraser, sticker, die, paperclip, etc.) on a separate sheet of paper (or on the back of this one) under the correct heading as being bigger, smaller, or the same size. Also, this makes for a great center activity!