To use a widescreen properly with DOSCab, you need to edit the aspect ratio in jukebox.ini. The default is 1.3333 for a standard monitor.
How do you figure out the correct value? Just divide the aspect ratio. If your monitor has a 16:9 aspect ratio, your aspect ratio is 16 divided by 9, or 1.7777777. NOTE: You want the physical aspect ratio, not the aspect ratio for the resolution you want to run in! DOSCab will adjust for the resolution aspect ratio automatically. Once you have the aspect ratio, set the MonitorAspectRatio setting in jukebox.ini to that value.
The skin you are using may not fit perfectly in the aspect ratio; if so, it will letterbox or crop the output. This is controlled by the FixedDimension setting (I couldn't think of a better name). FixedDimension = Height uses the full height of the skin, and crops or letterboxes the sides; FixedDimension = Width uses the full width of the skin, and crops or letterboxes the top and bottom. FixedDimension = Auto uses width for horizontal orientations and height for vertical orientations.
Of course, you'll also need to set the width and height settings in jukebox.ini to the LCD's native resolution. As long as your video card supports that mode in DOS, it should work. Some cards may need a VESA 3.0 driver installed to get unusual resolutions, others may do it in the BIOS. Not all video cards will have native support for widescreen resolutions, but if this is the case with your card you'll have the problem in both DOS and Windows.
Although you can use any skin with a widescreen display, the skin cd6-1-ws.skn is optimized for widescreen use.
--Chris