Click the Eyedropper Tool and set Sample Size = Point Sample
Click the Magic Wand Tool and set Tolerance = 30 and tick Contiguous
Click on a coloured area of your picture which is not too large
Note the size and; shape of the area selected
Press Ctrl D to deselect it
Now go back to the Eyedropper Tool and set Sample Size = 5 by 5 Average
Go back to the Magic Wand Tool and select exactly the same spot as before
You now know that the area selected by the Magic Wand is controlled by your setting in the Eyedropper!
Open a picture
Click on the Magic Wand and set Tolerance = 5
Change to the Rectangular Marquee and select a small area of the picture - one you will be able to repeat
Go to Select - Grow and note the area selected
Press Ctrl D to deselect it
Now go back to the Magic Wand and set Tolerance = 50
Change back to the Rectangular Marquee and select the same small area as before
Again go to Select - Grow and note how the area selected has increased!
Exactly the same applies when you use Select - Similar