You can set the summary view starting in The Bat! v6.2:
The main goal of the new view is to present all the messages’ summary in a compact and information-rich mode.
To switch to that view, you need to activate the option "Switch to summary view when narrower than" under the menu Options -> Preferences -> Message List. Then set the value upon which this option will come into action.
It is possible to set a fixed width of the folder contents view pane in pixels, or set a relative value which will be defined by the size of the preview area relative either to the program’s window size or the screen size.
Looking through the list of messages, you can quickly get an idea of what the content of particular messages are without invoking them. The Bat! is smart to omit greetings, quotes and display only the relevant text. To change the number of lines that are displayed in the message list, go to the menu Options -> Preferences -> Message List -> Color Groups, choose Generic Group and press "Edit". Set the needed number of lines on the tab "Single Column".
Contacts with photos will be visually identified more quickly than the rest, while other senders will get distinctive initials.