In your Attract Mode folder, you should see a file called: window.am
Open this with something like Notepad++ and add these lines to it:
position 100,100
size 320,240
...then save it.
Now launch Attract Mode and change the display to "window" mode (I think that's what it's called - it's been awhile).
That will now use the info from window.am and make a 320x240 screen of AM positioned at default X,Y of 100,100 on your monitor screen. You can move the AM window around and resize it like a normal Windows application window. When you do, it updates that window.am file to match so next time you launch, it'll use the new settings you made.