I have an issue with dosbox and was directed here...

I have a new wide screen monitor and when I do 'alt-enter' for full screen it does not look right.

It looks like everything is stretched out on the horizontal.
How do I fix this?

I want it to play like 'windowed mode' with the same aspect...I'm OK with the black stripes down the sides of the screen.

You can try the dosbox.conf settings from http://www.gamesas.com/index.php?/topic/1096895-how-do-i-decrease-the-ingame-resolution/page__view__findpost__p__16023191. Worked for me, but you may need to play around a little to get it to work on your system.

In the DOSbox configuration (if you're on Windows look in the start menu for it) you set aspect=true and set the fullscreen resolution to match your monitor.