hmm, have you tried editing the shortcut?
If not, here's what to do:
Go to you're FM.exe file and right click and drag it onto your desktop (creates a shortcut). You may have this already. Anycase, right click it and select properties. In the "target" box you should see: "C

Program Files\FM08\fm.exe" or something similar.
Add the following onto the end: -windowed -fullscreen -fullscreen_height=1200 -fullscreen_width=1800
Using whichever resolution you want (it's best to go for your desktop resolution). Start FM. Under options -> preferences uncheck fullscreen.
Hope that helps.