I read that:
In mathematics, magnitude or size of a mathematical object is a property which determines whether the object is larger or smaller than other objects of the same kind.

However, if you use the formula for magnitude on a vector3 position in the world, you’ll get a value that varies, how can the ‘size’ of JUST a position vary?

The magnitude in vectors is the distance between the origin and the result.
Basically, in roblox, the origin for vectors is (0, 0, 0).
If you have a vector3 (x, y, z), the magnitude would be sqrt(x ^2, y ^ 2, z ^ 2)