ruler
You don't measure with length. You measure the length of an object with a ruler. You would use inches or metres as your units.
mass of a substance or an object
You would measure the distance of the door with meters because liters measure volume not length.
centimeter
First of all, the buoyant force on the object doesn't depend on how you measure it, or whether you measure it at all. The buoyant force on an object in a fluid, whether it's submerged partially, entirely, or not at all, is equal to the weight of the fluid that would fill the space occupied by the object if the object weren't there.
You don't measure with length. You measure the length of an object with a ruler. You would use inches or metres as your units.
a ruler
hah
that would depend on the size of the object. for eg the length of the golden gate bridge and the length of a knife would take 2 completely different units.
To measure a foot length you could use a ruler, a yard stick, a tape measure, or any object that you knew was precisely one foot long on a side or edge.
Scientist would use milimeters.
You would use any unit that is for measuring length. In the realm of science, it would usually be from the Metric System, but you can measure volume in inches, feet, yards and so on as well. It would be the cubic form of whatever unit (cubic feet, cubic centimeters, etc).
mm is a measure of length, liters are a measure of volume, the two can not be equated.
Measure the three dimensions ( length, width and height) and multiply them together
There is no instrument in world which measure how small an object is but instruments measure how large an object is .
It's the length around a round object, like if you would run a tape measure around barrel or a ball.
Assuming the object is shape with opposite sides of equal length then the perimeter would measure 2 * 7 + 2 * 2 =18