config.res
+ Help - Which resoulutions do MultiFE support?

Just make sure you use a valid desktop resolution, which is required to have one of these format:
4:3, 5:4, 16:9 and 16:10 (all major display should been supported).
You should also NOT use example a 4:3 resolution on a 16:9 display, since it can look strected and awful.
Please Note:
Windowered mode is not supported anymore and is removed from MultiFE 2.10.