Why Is Single Celled Organism Have A Limit To The Size They Can Become?

I think the answer is more to do with surface area to volume ratio. If a single celled organism increases its size (surface area) by a scale of 2, the volume inside this cell will not increase proportionally, it will increase by a factor of (for example) 4. This is a problem for single celled organisms since they don't have the transport mechanisms that we humans have, for example they don't have a circulation system to transport oxygen and food products. Instead they absorb all the materials they need to through their cell membranes, from their environment. Since the volume of the cell (the bits that need the materials) has increased more than the surface area (the bits that absorb the materials) the cell can't get enough of these materials to satisfy its needs, and therefore will die.

So there is a limit to how big the cell can be, because at a certain point, the volume of the cell will be too large for the surface area.

The single-celled organisms can vary in size but still their sizes come in a certain range. Usually they are less than 1 micron (one-millionth of a meter) in diameter (in the case of the bacteria), to more than 100 microns (protozoans). The reason for a specific size limit is because one cell can only increase in length and width to a certain limit. The size of an organism usually increases because of the increase in the number of cells. Therefore, when a single celled organism will not divide then it's size will not increase significantly. A small increase in size will happen only because of the increase in length and width of one cell.