Resize product thumbnails in Global Product listings

I'm looking for a way to properly resize the product thumbnails in the global products display on the main site.

They default to 150x150 using and automated thumbnailing system, but there is no way to resize them. Doing it in css to anything other than a square just causes stretching.

Going into Products -> Store Settings -> Presentation on the main site to set the thumbnails in the Product List Settings has no effect. I want to try out different thumbnail sizes on the layout to see what effect it has on shoppers, but I can't figure out a way to do this.