Is this why we grow up? everytime a cell divides it gives two cells

the net increase in cells provides for tissue growth and once the rate at which cells divide is not as fast as the turn over rate we wither and die.
so if we somehow managed to make cells divide to only produce one cell then we would never grow up? :)

Yes, we grow due to the fact that our cells are constantly undergoing mitosis (cell division), and producing more cells.

For your second question:

Division is defined as the action of separating something into parts, or the process of being separated. By definition, after a cell division, we can't be left with one cell. If, however, you were to continue killing the cells you didn't want to survive, you could prevent an individual from growing. But why would you want to do that?