A giant represents primordial power, somewhat overwhelming in its force.
Psychological / Emotional Perspective:
Giants and ogres often represent the emotion of anger, particularly masculine anger. This can be confronted in dreams where perhaps we cannot do this in waking life.
Everyday Material Aspects:
Dreaming about giants may mean we are coming to terms with some of the repressed feelings we had about adults when we were children. They may have seemed larger than life or frightening in some way.
You might also like to consult the entries for Anger, Monster and Size.