Main Image Width not working?

Hi, I really like your plugin, but I’m having a strange issue. I assumed that the option “Main Image Width” in the item posts layout changed the width of the image that shows in the Expander. But it doesn’t actually change anything at all (as far as I can see).
I want to be able to make the main image in the expander a little smaller, and have the right-hand column (I have it set to a 2-column layout) be a little wider. How can I do this, if the “Main Image Width” slider isnt working?