25.4 mm makes 1 inch so a circle 3 mm diameter means it is 3 millimeters from one side to the other, which in inches is 0.118 inches or 1/10 th of an inch on your average ruler. Similarly 6 mm is equivalent to 0.236 inches or a little over 2/10 ths (tenths) on the ruler. thus a circle of 3 to 6 mm in diameter would be about 1/10 to 2/10 inches from one side of the circle to the other side, along any line that splits the circle in half. A circle of 2/10 ths of an inch diameter (equal to 6 mm) would be approximately similar to the circle at the end of your average yellow pencil before you sharpen it. Hope that helps you out.
3mm is about 1/8 of an inch, 6mm is very roughly a 1/4
The end of a pencil should be about the right size
A dime has a diameter of approximately 18mm, so you should be able to fit 3 to 6 of these circles in a line and it would fit inside a dime.
Dime is definitely too big. If you have one of those old school orange pencils that require sharpening (the number 2 ones), the eraser on the end of that is probably about the right size.
I always reckon that 1mm is about the width of a pencil lead or the point on a ball point pen.
Diameter is the distance from one side of a circle to the other, passing through the centre.
I'm not sure the exact size of a dime, (being English Ha ha.) but any coin I can think of would be much bigger than 3-6mm.
Hope this helps