Cells are so small so the surface area and volume of them can be proportional to each other. This helps with the efficiency of the cell's absorption and waste expulsion processes. Also by the cell's smallness, communication from the nucleus to other organelles is fast and the cell can be regulated while the conditions for diffusion are still ideal.
Cells are small because they need to be able to diffuse through materials easily. Also, materials inside and outside of the cell need to easily pass through the cell membrane, which makes it harder and slower for the materials to pass through the cell membrane
Cells are small because the ratio of surface area to volume is the most efficient at their size. This is because when you have a lot of small objects vs a few big objects the surface area is much larger in relation to volume. Another reason is when the cells are small and plentiful, they are easier to replicate and replace when damaged.
If a cell grows beyond a certain size, materials will not be able to pass through the membrane fast enough to accommodate the inside of the cell. So when the cell reaches a size too large, it will divide into smaller cells in order to maintain a surface area/volume ratio that is more favorable to the functioning of the cell. Therefore, cells remain small in order to survive.