I cannot seem to figure out how to get vertical games to display properly on my LCD monitor.... I just can't seem to get the game to take up the entire screen and display at the proper ratio.
(Looks like) You are using a widescreen monitor (10:16). Pacman runs at 3:4. So you can't have both the game "take up the whole screen and display at the proper ratio" at the same time.
The game is running at the correct ratios as is.
If you are more worried about filling the screen, and you don't mind vertically stretched pacman and everything else, disable keepaspect:
-[no]keepaspect / -[no]ka
Enables aspect ratio enforcement. When this option is on, the game's
proper aspect ratio (generally 4:3 or 3:4) is enforced, so you get the
game looking like it should. When running in a window with this option
on, you can only resize the window to the proper aspect ratio, unless
you are holding down the CONTROL key. By turning the option off, the
aspect ratio is allowed to float. In full screen mode, this means that
all games will stretch to the full screen size (even vertical games).
In window mode, it means that you can freely resize the window without
any constraints. The default is ON (-keepaspect).