I am using a Page Content Rendering with a custom variant to output an Image property. How could I control the dimensions of the raw image output.
There is no variant definition for an Image field (like there is for a Date field). This is definitely something that is missing in the current version of SXA - I do expect that it is on the roadmap though.
So with simple variants, I'm afraid you cannot scale the image (you could use css to control dimensions but that is not what you want to do).
You might be able to do this using NVelocity, but that will not be as user-friendly as a 'normal' variant definition.