To dream of the size of something represents how important you feel something is. A reflection of how powerful, competent, or dangerous you feel a situation or person is exerting. A reflection of you feel you "measure up." Your feelings about how big of a deal a problem is.
Big sizes may reflect importance, dominance, or the danger of being overpowered. Feeling that an issue is the most important or too dangerous to be ignored. Feeling that something is impossible to control or defeat. A sign that your anxiety or insecurity may be magnifying a problem in your mind. Your personal history or sensitivity may be making a problem difficult to tolerate. Feeling that problem is growing.
Small sizes may reflect feelings of being trivial, impotent, or the belief that a situation is easy to control. Something that doesn't feel threatening. Feeling annoyed by a small problem.
Medium sizes may reflect feelings that something normal, average, or "just right".
Example: A young boy dreamed of being very small and almost being stepped on. In waking life he was a person with a very small ego and was afraid of being treated as a trivial during a family fight.