You could use a simple ruler, or a micrometer for greater accuracy.
Meter.... or more likely millimeters
depends how big it is
You measure it. Any millimeter graded ruler or measuring tape or yardstick would tell you the diameter, but the thickness is most easily and accurately measured with a vernier caliper.
Ok so if you have a tape measure open it and I will show you how it is used exactly. So as you can see at the top which are the big lines of the tape measure they are called (inches). This is used for measuring the formula of a arm size, hand size, hip size and the waist size. But now I will talk about measuring as assessing width as my measuring unit. So basically the top constituent part of the tape measure are the inches and the bottom are the cm's. So to find the width would mean that every 10 cm is the width but when go on to a quarter of width it will basically mean that the answer would be 1w and a quarter.
The millimetre.
You would measure it in centimeters.
Measuring tape.
You should clarify what you want to measure about it: its diameter, mass, weight, color, etc. If you want to measure its diameter, that would usually be specified in microns (= micrometers).
I would find a ruler, a tape measure, or a micrometer, and use it to measure the maximum distance between any two points on the quarter's circumference. Having that number, I would then examine the tool to determine what system of units forms the basis of its graduations. If that system is any other than the English system of units, I would then apply the appropriate conversion factor that relates the English units of length to those of the system indicated on the tool.
Calipers are used to measure the dimension of anarticle. In measuring a section of pipe you would use an inside caliper to give the internal diameter, and an outside caliper to give the external diameter. The difference being the thickness of the pipe wall.
a tape measure
measuring tape