EDIT: I've had a little look at whether we might be able to add an option to remove it/display it differently based on user preferences. This may be do-able within the forum/tracker/user blogs and if demand is great enough then I'll certainly consider working on it The green bar on the main content site would remain though.
I'm sorry, I'm not quite sure what you mean by the 'new posts banner', but the templates for these areas of the forum are shared between multiple listings (search results, view new topics/active topics/unanswered etc etc) - this is how the forum software itself works. So, while it may not look brilliant if there are only a few posts listed, we feel it looks nice the majority of the time.
Reducing the width of your browser window will adjust the display mode to tablet or mobile sized (you may need to reload when you do this, to force the advertisements to resize... the Google ads only seem to be smart about that when it comes to actual screen size changes rather than resizing the browser window).
May I ask what device/browser you are using please? I haven't spotted the varying width of the last post column? I'll take a look at that as they shouldn't be varying that much! Whereabouts is it that you are finding the font too small? Thanks.