What determines the size and shape of a cell?
This is quite a hard to answer question.
There are different cell cycle molecules that, if mutated or knocked out, have shown to cause the cell to have a different shape or "overgrow" in size or not grow at all...

You might find some insight here.

There are lots of factors that influence cell size and shape, thus it is a very complex system to study. For example, cell shape is influenced by the cytoskeleton but also other factors.