Answer:
For the first question, you have to first understand how cells work. Cells transport nutrients and water through their cell membrane, and it's their only way of transporting things they need in and out of their interior. Hence, a larger surface area would be best for cells to survive.
You might ask, but then why wouldn't it be better for cells to be bigger if a larger cell membrane is beneficial to their survival? After all, as the cell gets bigger, their cell membrane gets bigger too!
Unfortunately, you'd be forgetting one thing: the insides of the cell, otherwise known as the volume of the cell, grows too. So, it becomes a mathematical question at this point: imagine a cube. If you doubled it's side length, your surface area would grow by 4 times (), and your volume would grow by 8 times! ()
Hence, if the cell got bigger, it's volume would grow faster than the surface area, and the cell would start to build up carbon dioxide and other waste materials.
This is the main reason why being small helps cells survive. Of course, there are many other reasons, like compactness, efficiency and survivability, but this is the main reason.
Of course, if you're smart, you'd have already discovered one thing: this is exactly the reason for your second question as well. Since a large organism being made of one or few large cells would greatly decrease surface area-to-volume ratio, the efficiency would plummet and the large organism would die. Hence, many small cells would help large organisms maintain homeostasis.
Explanation:
Hope this helped!