Why Cells are (need to be)
Small?
First reason = They need the interact with environment, which is easier for them
if they are small.
Second reason = They have to diffuse for survive.
For instance, nutrients, O2 and extraction of CO2. They cannot make efficient
diffuse if they are bigger, due to surface area to volume ratio.
Third reason = It would make them slower and might not be able function as they
do before, as they need much energy and time for moving things across the cell,
which will effect the speed of chemical reaction.
Surface Area to Volume Ratio
Image a cube with dimensions of 1x1x1, and calculate
the volume and surface area of that cube.
The calculation of volume is 1x1x1 = 1 and surface area
is 1x1x6 = 6, so the ratio would be 6.
Image a second cube with dimensions of 3x3x3, and
calculate again the volume and surface area of that
cube.
The calculation of volume is 3x3x3 = 27 and surface area
is 3x3x6 = 54, so the ratio would be 2.
It is clear in the results that the Ratio is going lower if you make the
surface area and volume bigger, but unfortunately surface area
cannot increase as much as volume (because volume increases as
cube while surface area increases as square).
When you triple the dimension of a cube, the volume become 27
times bigger while the surface area only increased as 9 times.

So, when volume increases, the cell’s needs of reactants and molecules increases
as well but surface area cannot keep up with this increase, which leads that
transport of a molecule would not be sufficient enough to reach the cell’s needs.
That is why most cells need to divide, to decrease volume and increase the
diffusion speed.

But some cells can increase their surface area by having folds in the cell
membrane, and have more interaction with the enviroment. Such as intestine
cells.

In SCJAS (South Carolina Junior Academy of Science)
it also mentioned the effect of the Ratio of Diffision as well in Agar Blocks

In a project that they gave for their students as a presentation homework,
they mentioned in their results as «The results of the experiment supported the
hypothesis because the cubes with the largest SA:V ratio had the quickest rate of diffusion»