Hi Vipin and welcome to the RH forums.
You can control your CHM Window size and position via the Postion section of the window you are using in your Single Source Layout (see below). You can open your window by double clicking it in the Project Set-up tab. If it doesn't look like the one below, don't worry. Just right click on the Windows folder, select New Window and select the Microsoft HTML Help option.
In addition to what Colum advised, note that you aren't forced to just use the values in the fields. You may click and drag the window in the large gray preview area to adjust placement. However, note that because CHM is an older format, an area of 1024x768 is assumed here.