Proportion is about the size of things when you compare them with each other, for example when a girl is half the size of her mother. Proportion has big effects in art, mathematics and everyday life. For example, if you ate a 200 g bar of chocolate you might feel sick because it is four times the size of a 50 g bar. Another example is a map drawn to a scale of 1:100,000 where 1 cm on the map is proportional to 1 km in the real world. Measuring helps you understand proportions better.
Part of the effect of art works is their scale. This is one reason why it is important to see works of art in reality, not only in photographs. They might be much bigger than you think. They might be very different in size from the original subject. Ideas about proportion can also be used in composition to create harmonious effects.