Explain why cells are small instead of large

In a technical term, there is both small cells and large cells, but they are microscopic because they are what organic life is created upon. With cells, living creatures is possible. Cells are also small since there is such a large quantity of them, that they can be combined with other cells into a conglomerate that will synergize into a life form.

Also cells can be big, for example the egg cell within a female human can be as large as the head of a pin, which is absurdly large for a cell.

If it doesn't have a nucleus its a prokaryote

Cell division is important to single celled organisms because it allows the cells to reproduce. through cell division, cells make exact copies of themselves. when single celled organisms go through cell division, they make exact copies of themselves which is how they reproduce.