The main reason that cells are small has to do with how the ratio of volume to surface area increases as a cell gets bigger. One way to think about this is imagine a cell as a water balloon. If you cut the balloon open and lay it flat and measure the area, that measurement would be the surface area. The volume of water inside the balloon would naturally be the volume. If you fill the balloon with a little bit of water, the ratio of volume to surface area will be low, but as you add more water, the ratio of volume to surface area will increase. This is because the balloon is only stretching a little bit more to accommodate a lot more water. The implications of this for cells is that all nutrients have to pass through their cell membrane which is only on the surface. These nutrients then need to get to the center of the cell. If the cell has too much volume, the nutrients, such as oxygen, may be depleted before they get to the center of the cell. Therefore, cells generally don’t get too large because they would have trouble transporting nutrients and other molecules from the outside to the entirety of the inside.

That being said, there are exceptions and there are cells that are quite large that employ different strategies to overcome this difficulty. An example is egg cells which can be fairly large (such as the ostrich egg) and neurons that can be very long (especially in the case of the giant squid).

First off all, not all cells are small! Sure, most of the cells are small because they need to transport tiny cargo in the form of proteins or lipids and, most importantly, messages from the nucleus of the cell, which is like the command center, to everywhere inside the cell. Therefore it would take a really long and inconvenient time if they were really large.

But not all cells are small. Let's take a look : red blood cells are about 5 microns across, cells below our skin are about 10 microns across, fat cells which are full of fat are 100 microns, and a the largest neuron cell we have can be up to one meter (40 inches) long ! It really depends on their function.

Interesting question! In fact, cells come in many shapes and sizes. For example, one of the longest cells in the human body is a neuron that extends the entire length of your leg! One of the largest cells you will find in nature is the oocyte; the female contribution to the embryo. A chicken egg is a cell! So the largest cell in nature is probably the ostrich egg.

Biologists are very interested in what determines cell size. Is there a limit to the size a cell can be? What constrains cell size? Do bigger animals have larger cells or just more cells? Keep asking questions!

The reason cells are so small is usually explained in terms of surface-area-to-volume ratio. The larger a cell becomes, the lower this ratio becomes: surface area increases much slower than volume. This becomes a problem when the cell can't get enough food (based on its surface area) to supply to all of its interior volume. So cell size is the best compromise between having resources and being able to supply resources.

It's because of the square-cube law.

Think about this: suppose you have a cube of something, and you want to get chemicals in or out of it. The only place to get in or out is on the sides of the cube. Each of these sides is a square of length and width equal to the length of the sides, squared. Thus, the total surface area of the cube is six times the square of the length. If the cube's length is one micrometer, let's say, then the cube's surface area is six square micrometers. If the cube's length is two micrometers, then the surface area is 24 square micrometers.

However, the mass inside of the cube is not proportional to its area, but to its volume. This is equal to the cube of the length. Our one-micrometer-long cube has a volume of one cubic micrometer, and our two-micrometer-long cube has a volume of eight cubic micrometers.

Notice that the ratio of area to volume is not the same between these two hypothetical cubes! The one-micrometer long cube has a surface area/volume ratio of 6/1 = 6/micrometer. The two-micrometer-long cube has a surface area/volume ratio of 24/8 = 3/micrometer. This means that if we need to get nutrients or something into the larger cube, we can only do it half as fast!

As a consequence, cells have to be small, in order to maximize their surface area/volume ratios. If they were bigger, they couldn't take in nutrients fast enough, and they would starve to death.