what do you mean by pixel size? there are several concepts about pixel size.
As we zoom-in and zoom-out the image the size of pixel changes.
pixel size in an image that is printed on a paper is constant and doesn't change.
if the camera is fixed and image is also fixed then the pixel size doesn't change.
for an image of size 640x480 (pixels), the width is of 640 pixels and height is 480 pixels. if the background doesn't change we may go for a conversion such as DPI (Dots Per Inch) or PPI (Pixels Per Inch).
suppose the background size is 64x48 inch, then the DPI = 10 and Pixel size is 1/10 inch.
Hope this is clear.
If we zoom-in the picture and the background size reduces to 32x24 inch, then DPI = 20 and Pixel size is 1/5 inch.
If anyone can provide a better explanation or wants to correct me are welcome.