Use a magnifying glass and look at your monitor, you will be able to see the pixels (dots).  It has close relationship with the “resolution”.

The following are from what I have understood, ignore the following if you do not like its technical part.

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If a picture of size 10px X 10px is displayed on the screen at 100% (no zooming), it is supposed to take 10 X 10 pixels on the display.  I do not believe that every screen is always 72dpi (dot per inch).

For printed pixel, if it is 300dpi, it means that there are 300 dots on an inch, the size of the pixels must be the same or smaller than 1/300 inch.

On the same monitor, if we increase the screen resolution, the pixel size will become smaller so that the monitor can display more dots.

On different monitors but with the same resolution, the larger screen has larger pixel size.

I can tell you the pixel size on a monitor or printed material, but I cannot tell you the pixel size of an image file without the output, because it depends on how big is the output.

Some picture files has the dpi properties, but it is only a relative reference so that it can tell the user how big it is in cm or inch.  The pixel sizes might be different if the output has different dpi.  Therefore I always use pixel rather than inch or cm when handling images.

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Pixel can be as big as a human being.  It happens when a large group of people wearing different colour of clothing lines up together, words or patterns can be seen from far away.  It can usually be seen during an opening ceremony (such as the Olympic Games) or other important celebrations.