right now, if you want to increase the image size, you need to edit the source code. It's /home/pi/pimame/pimame-menu/pmmenu/romlistscene.py

lines 54, 55, and 56

self.boxart_area = pygame.Rect(0, self.info_container.top, self.info_container.w / 2, int(self.info_container.h * .4))

self.info_box = pygame.Rect(0, self.info_container.top, self.info_container.w / 2 - padding, int(self.info_container.h * .4))

self.description_area = pygame.Rect(0, self.info_container.top + self.boxart_area.h + padding, self.info_container.w, self.info_container.h - self.boxart_area.h - padding)

each line is specifying a rectangle(left/x position, top/y position, width, height).
self.boxart_area-> boxart
self.info_box->game info (number of players, file-name, etc)
self.description_area->game description
self.info_container->container for all 3 other rectangles

so right now, self.boxart_area and self.info_box are set to both be half the width of the container and 40% (.4) of its height
since it's tall and skinny now, you could change them to something like this:

self.boxart_area = pygame.Rect(0, self.info_container.top, self.info_container.w, int(self.info_container.h * .333))

self.info_box = pygame.Rect(0, self.boxart_area.top + self.boxart_area.h + padding, self.info_container.w, int(self.info_container.h * .333))

self.description_area = pygame.Rect(0, self.info_box.top + self.info_box.h + padding, self.info_container.w, self.info_container.h - self.boxart_area.h - self.info_box.h - (padding*2))

The above code has not been tested, but barring spelling errors, should work to make all areas the full width of the container and ~ 1/3 the height of the container

yep. at some point, I plan to implement an easier way to edit those, but we have a lot of other problems that we need to get fixed first. :D