When you describe a quantity with more than one familiar unit (such as kilometers or millimeters for a length), how can you decide which is a more appropriate unit to use? while it is not incorrect to use the less appropriate unit, why do you want to avoid using it anyway?

It depends on the scale. For example, if you want to express the length of distance between 2 towns, it would be appropriate to use kilometers or meters rather than millimeters. The value of millimeters would still be equivalent to the same amount of kilometers. However, you should report it to the appropriate scale that is easily relatable when you want to measure it qualitatively.