Place items of the same shape, but different sizes, on a table or other child-friendly work surface. These items could be plastic beads, pom-pom puffs or blocks. Use only five or six items at a time to begin with, so the child does not feel overwhelmed by the task.
Ask the child which item to choose the smallest thing in the pile. If she is unsure about how to tell size, show her how to compare one item to another to determine the smallest shape. Set the first shape to the side.
Move on to the next smallest shape. With the first item put aside, ask the same question again about which piece is the smallest in the pile. Then set that one next to the first one. Continue this process until all the shapes have been sorted.
Look at the lineup of shapes that are now sorted according to size. Talk with the child about how things look as they get bigger or smaller. Ask him about what he sees, and have him describe it to you.