Größe
... is a German word with different meanings
a) It can simply mean height or bigness.
b) It can mean size as in the size of clothes.
c) It can also mean a property that can be measured, a quantity
It's the third meaning that is often used in maths. A quantity (Größe) in maths and physics is some property of a thing that can be expressed in numbers and which can be measured. Here are some examples:
-> length
-> height
-> duration
-> speed
-> price