In my (admittedly limited) digging, it appears the resolution options are hardcoded into the EXE or one of the other game files. I cannot find an editable file or registry string that will allow true widescreen resolutions... yet.
I got it by adjusting the resolution using the in game settings. I think the max res was 1600x1200
1600x1200 is not widescreen, that's true 4:3 resolution, not 16:10 or 16:9. Some monitors will stretch the horizontal or crop the vertical on 1600x1200 to widescreen because the vertical res is beyond the monitor's physical limits (if your monitor maxes out at 1080, it can't actually do 1200). Either way you are not getting an actual widescreen res, you are either getting stretched/squished visuals or losing the top and/or bottom of the entire image.
Yeah, I believe the in-game settings allow you to max out the resolution to match whatever your screen's native vertical resolution is, but it'll always be in 4:3 aspect ratio, so you'll have those unsightly black bars on either side. This may simply be something that someone will have to make a patch to get working, which seems unlikely at this point. In my Googling, I did come across someone who had posted super high resolution screenshots from the game on some graphics showcase forum, but I think they may have been running the PS2 or Xbox version through some kind of emulator. In any case, they didn't say anything about how they achieved those visuals.