square yards
centimeter
square centimeters
As square inches are a measure of area, you can measure any area you like in square inches. However, depending upon the size of the area: square feet may be more appropriate for the size of a room square yards may be more appropriate for a small piece of land acres may be more appropriate for a larger piece of land square miles may be more appropriate for a very large piece of land.
As square meters are a measure of area, you can measure any area you like in square meters. However: for large areas, eg the area of a piece of land, hectares or square kilometers may be more appropriate; for small areas, eg the area of a piece of paper, square cenitmeters or square millimeters may be more appropriate.
The measure is the area, which is found by multiplying the length by the width using an appropriate unit of measure. For example: an area measuring 12 feet x 3 feet = 36 square feet.
square centimeters
centimeter
The appropriate measure would depend on whether you wanted to measure its circumference, area, volume, temperature or some other attribute. For this reason, it is not possible to name an appropriate unit.
The metric unit of the meter would be the appropriate measure of the area of classroom. Calculating area involves multiplying the length (in meters) of the classroom's length and width. Area of the classroom would be in square (^2) meters.
Square centimeters
square centimeters
Metric
50*50 ft = 2500 ft is a measure of length, not area. It would not be appropriate to convert a length into an area. 50ft * 50 ft, on the other hand, is a measure of an area and is equivalent to 232.26 sq metres (to 2 dp).
As square inches are a measure of area, you can measure any area you like in square inches. However, depending upon the size of the area: square feet may be more appropriate for the size of a room square yards may be more appropriate for a small piece of land acres may be more appropriate for a larger piece of land square miles may be more appropriate for a very large piece of land.
Any unit of area, such as square meters, hectares, square kilometers, etc.
It depends on what you're measuring it for. You could be testing it for the following measurements: Height (using meters would be the most appropriate). Length (using meters would be the most appropriate, kilometres if long enough) Mass (tonnes would be the most appropriate). Volume (m3 would be the most appropriate). Area (m2 would be the most appropriate). Time taken to cross (depends on the length of the bridge. Minutes and seconds would be appropriate).
Metres or square metres - depending on whether you wanted to measure its linear dimensions or its floor-space (area).