>I need to design a form and its size should be 1920x1080 but in fox designer i can set the maximum design area to 1600x1200....Is there a way to set a different maximum area ? The problem is that i can't design this form at runtime and i really "design" it and use all the available area.....

No, it's unlimited. Tools->Options->Forms. Set design area to [none] and press Set As Default.