No ini file, sadly. I'd have tried that already had I found one.
There's the usual .dat file every KOEI game has had so far.
Koei's MO is to use a counter in the save file to link to different hard-coded resolutions. For example an "0" would be 640x480 and a "1" would be 800x600 and so on.
The resolutions list for DW6 is below. The game is 16:9 anamorphic so the left column is the viewable area and the right is the actual resolution.
640x360/640x480 4:3
800x450/800x600 4:3
1024x576/1024x768 4:3
1280x720/1280x720 16:9
1280x720/1280x768 15:9
1280x720/1280x960 4:3
1280x720/1280x1024 5:4
1600x900/1600x900 16:9
1600x900/1600x1200 4:3
1920x1080/1920x1080 16:9
1920x1080/1920x1200 16:10
What is really sad is that they have every 1280 resolution under the sun except for 1280x800.
However, if your system supports aspect scaling you can get a fairly acceptable fix for your system. Just have the 1280x720 resolution scale up to a letterboxed 1440x900 with a 1440x810 viewable area.
There are a bunch of tutorials on this site if you need help.