I have a parent gameobject that is basically just an empty, it has a child which is a building prefab. Is it better to scale the parent up in size, or the child element?
Yes. 
It kinda depends what you mean by ‘better’.
Generally, scale just what you need to scale, eg, just the pieces.
But sometimes you need to scale more, eg, the entire object.
Also, NEVER scale things with Vector2() because the z term is zero and may cause much confusion.
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If it’s just one object, you could do either. If you child more of them, you can scale the parent which will scale them all, plus you can scale children individually to be larger/smaller relative to each other.
The issue that comes when setting parent scales is that if you then needed to set a child item to a known lossy (world) scale, it will not be straightforward clear what local scale you need to set the child item such that it has the desired lossy scale because the parents have non 1,1,1 scale. (Note: you cannot set the lossy scale of gameobject in code, only the local scale). Now, if you leave the parents at 1,1,1 scale, then the local scale of child item will be equal to its lossy scale. So it makes it easier to work with when just setting the child scales.