Are there some (maybe not so) hidden options anywhere where I can set the default rendering size for molecules in tables? Maybe even choose another renderer instead (as default)?
The Renderer to Image node will probably do what you are looking for. It let's you choose a renderer/resolution for images for a final output. I don't think there is another way, but that would be a good question for the RDKit specific section on the forums.
If you can use the Marvin Renderer, you can set the default size in the Marvin Renderer Preference.