You could use a simple ruler, or a micrometer for greater accuracy.
Meter.... or more likely millimeters
A micrometer or a caliper would typically be used to accurately measure the diameter of a crankshaft journal. Both tools are precise and can provide a reliable measurement of the diameter.
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.
To measure a seedling, you would typically use a ruler or a measuring tape for accuracy. For smaller seedlings, a ruler may suffice, while a measuring tape is better for taller or more flexible plants. Additionally, you could use a caliper for measuring the diameter of the stem, if needed. Make sure to measure from the base of the seedling to the highest point for a consistent height measurement.
To find the density of a quarter, you would first measure its mass using a scale. Then, you would measure its volume by water displacement, where you would measure the amount of water the quarter displaces when submerged. Finally, divide the mass by the volume to calculate the density of the quarter.
The millimetre.
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.
That would depend on what fruit it is exactly, apple, orange, what? Plus even individual fruits are not all the same size, such as two apples, for instance.
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).
You would measure it in centimeters.