"My hand is bigger than your hand" or "My cookie is smaller than your cookie". Try to incorporate "bigger than"/ "less than" vocabulary in your routine today. Show your child the <> signs.
Place an apple slice on a piece of paper, then place bigger slice beside it. Draw a box between the slices. Let your child draw the correct "bigger than or smaller than" sign in the box. You can talk about how one apple piece weighs "less than" the other, and one apple piece takes up less volume than the other.
Place 4 raisins on a piece of paper and then place 1 raisin beside it. Draw a box between then and let your child decide which sign ("less than or greater than") should go inside the box.
Check out this site made by the UofU comparing the sizes of a coffee bean to a red blood cell and ultimately, to a water molecule! This is a fun site to see size comparisons. It even shows a graph box disected to emphasize the miniscule nature of these objects. http://learn.genetics.utah.edu/content/begin/cells/scale/