The width of a fingernail is about 10 millimetres, which is one centimetre. One more way to think about how big a centimetre is is in terms of inches. About three times as small as an inch is one centimetre.
In contrast to the U.S. or imperial system, the metric system is used to measure a centimetre. The most common tool for measuring distance is a ruler, which has lines that are marked in centimetres or inches. This makes it easy to compare the two systems. You might also find it helpful to compare the size of a centimetre to common things. Orders of magnitude, or the scale of relationships between numbers, are a more precise way to do this.