"Sheet size" refers to the size of the sheet that a print is on. "Image size" is the size of the printed image, contained within the sheet. If a client orders an A3, for example, we will assume they are referring to an A3 sheet of paper, with the printed image slightly smaller then A3, with margins on each size, as per above illustration.
"Paper margin" refers to the space between the image edge and the sheet edge. Generally this is unprinted and you are therefore looking at 'paper white'.