For the ATW Show Posts, the FI is matched to Weaver II/Aspen/Weaver Xtreme FI sizes. That can’t be changed because of the mechanism used to display the posts – which is actually just using the existing code to display posts in the themes.
Weaver Xtreme has a bunch of new options for displaying the Post FI size based on excerpt, full, or single page view. This is as close as it can get.
As for the [show_posts] plugin, it will display FI’s as a thumbnail by default – IF the “Disable sutomatic post display integration” option is checked. But then the posts will get a farily generic display that won’t match the theme’s display.
There simply is no technical way to have the [show_posts] option display any different size FI while at the same time integrating with Weaver themes.