First, you can adjust your Messenger window by grabbing and pulling the sides to widen it and corners to lengthen and/or widen it.

You can also change the size of display images and details with the…Contact Detail Slider! Open it in the upper-left corner by clicking the little set of horizontal bars icon. Slide left to decrease size. Slide it back to the right for BIG FRIEND FACES (or whatever their display images might be… ;o)