the outside of the shape
Displacement
If you wanted to describe the length and breadth of an irregular shape you would find the maximum length and breadth. You could sandwich the object between two flat boards, and measure the distance between the boards. Then rotate the object 90o and do it again, for the breadth.
If you wanted to use English Units you would want to use feet or inches. If you wanted to use the metric system you would want to use centimeters.
You would use square inches if you were trying to measure the area of a 2D shape.
It depends on what shape the 2.1 acres is. However, if the shape is a square, then each side would measure 302.45 feet.
energy release
Grams is a measure of weight so you would not use that. If you measure the depth, width and length of the pool in metres, you will be able to calculate the volume in litres.Every stere (cubic metre) is 1,000 litres.
Personally, I would measure the length of a grain of rice in millimeters.
I would assume that you would use cm or even mm if you wanted to be more accurate
Refuse to report the measure - Apex
A centimetre, unless I wanted a very precise value (why?) in which case I would use a millimetre.
It depends on what object you are trying to measure. Displacement is appropriate for a medium sized, non-soluble object, of any shape. It would be pretty useless if you wanted to measure the volume of a concert hall or a needle! And it would not work for a sugar cube either. A micrometre would be appropriate for small objects with simple geometry, such as a needle or possibly a sugar cube. But is would be no use for a concert hall nor an irregular object. A ruler would be appropriate for large objects with a simple geometry such as a concert hall. But it would be no use for a needle, nor for an irregular object.