Use the information in the BGWiki Graphics Guide to access FFXI's registry.
Set MIP mapping to 4
Set Screen resolution to the size of the window you wish to use
Set Background resolution to 2x the Screen resolution size
Set environmental animations to smooth
If you're using Windower, you can enhance the game's look further by pressing F11 to manipulate ambient lighting. This will make the game darker, but will give models a more natural appearance.