* Penny: US pennies are 1.9 cm in diameter, so 1.5 cm is slightly smaller.

* Thumb: The width of an average adult thumb is about 1.5 cm.

Here's a visual comparison:

* Small: Imagine a line drawn on a ruler from the 1 cm mark to the 1.5 cm mark.

* Medium: Think of the thickness of a typical credit card.

So, 1.5 cm is a fairly small measurement.