What do cells need to do between divisions to make sure they don't just get smaller and smaller.

The answer is: cell grows in size. Before mitosis begins, cells must double their genetic material, grow in size, and double other organelles. The growth of the cells as a preparation for division occurs during the G2 stage of interphase. Cells will grow until mitosis begins. This way it is ensured that cells don't get smaller and smaller from one division to another.