Responsive Map - How to set the dimensions on the static version?

Hi Guys,

When using the map field type and creating a responsive map (based on the docs). We have to remove the width and height values, which I think are used to generate the fallback static image.

Without these it seems to be generating this static image at 460x320px - how or where do I set this to control the size this static image is created but to keep the map responsive (no width/height values).

Thanks Terry