Thats not how lcd's work. They always and only do native resolution. Best output is to have mame do the stretching and have the output be the lcd's native resolution. Why? otherwise you have some software stretching, then some hardware stretching... Stretching a stretched image sucks.
Anyway, it isn't mame... its the monitor
> I've been using MAME on and off and recently I downloaded vmame64 and noticed
> that it's doing an awesome job of scaling the video output; I had never been
> able to get it working this good in the past so I'm not sure if some code was
> updated. I'm running Vista at 1680×1050 and MAME is scaling the game to fit a
> full-screen 4:3 area but the pixels aren't square. I'm not using any filtering
> and I'm using D3D and I've tried various settings. I figure that since I'm not
> running at the actual native resolution of the game that it'll never be perfect.
> It's CPS1/2/3 and NEOGEO games that I can easily see this on. In the past I was
> able to get 384×224 out of my video drivers on a Sony 17" CRT monitor but has
> anyone had luck getting these resolutions to work on an LCD?