Size: size is how big or small something is. It is the actual dimensions of the thing. A house can be big or small, an aircraft carrier is very big.
Scale: scale is the relationship between the size of a thing and us, it is the perception of size. It is, in a sense, size using a person as the basic unit of dimension. So scale is tricky. You need some clue or some indicator of how large or small a thing is. This can be a person
or an object with which we are very familar with, something whose size we know
Here in the West, we who grew up in the East are often surprised by the panoramic scope of the landscape. It seems to stretch horizon to horizon without houses or roads or even trees to indicate the size of the landscape. In the eastern forests, dense with vegetation, you can only see for great distances when you climb to the top of some mountain or hill. Then the view is expansive but inevitably includes some signs of human habitation, some houses or church steeples or some sign to lend scale to size. In the West, the relative barrenness of the landscape deceives the unfamiliar - distances are significantly greater than they seem because there is often nothing to gauge scale to size - no buildings, no roads.
it is often hard to tell how near or far off a peak or rise is until something can be included to provide scale
"Size" is mathematical, "scale" is anthropomorphic. All buildings have size, but not all have scale.