Living things come in a huge range of sizes, from a bacterium about 1 µm long to a whale many metres long. Scale tells you how many times bigger one thing is than another. To compare two sizes, put both measurements in the same units and divide. A plant cell 100 µm long is 100 times longer than a bacterium 1 µm long.
An estimate is a quick, approximate answer. To estimate, round each number to one significant figure and then do the calculation. For example, 48 × 205 is about 50 × 200 = 10 000. The size of an answer in powers of ten is called its order of magnitude, and an estimate tells you this quickly.
Use an estimate when an exact value is not needed, when an exact measurement is impossible (for example, counting the cells in a whole leaf by counting the cells in one small area and scaling up), and to check that a calculated answer is sensible. A check like this helps you spot a slip such as a misplaced decimal point.