what are two reasons why cells are small?

Answer 1

The first and most important thing is for the surface area to volume ratio. The smaller the ratio, the more efficient the cell.

Also, cells are building blocks of something bigger. If we had huge cells, imagine how big we'd have to be in order to hold trillions of them. ;)

That's about it in the nutshell.

Answer 2

1. THere's a limit to the ratio between the area of the cell membrane and the cell's volume.
2. There's a limit to the ratio between the nuclear material and the cell's volume.

Beyond these limits, the cell divides.
(exerything stated here are only theories)
For exceptions to these theories, see the movie(s), "The Blob" !

Answer 3

so that they have more surfaces area