Hi Raj,
is this an issue only when you disconnect the big monitor? what happen when you resize the browser window after the discunnect? I assume that your issue is caused by missing event from the browser to design studio when the monitor disconnect causes the size change. In this case, the solution will be difficult.
you can try to use the Design Studio SDK: Client Information Component from scn community package and check in some test scenario first if the size is correctly reported after disconnect.
Karol