如何在bootstrap中获取窗口的高度和宽度

In bootstrap I have a fixed top nav bar and fixed bottom nav bar. I want to show a large image in the background between the space of those two nav bars and I also want to cover the width of the window. How can I dynamically get the height between the navbars and the width of the window? The window size may change depending on device.So I need it dynamicIn bootstrap I have a fixed top nav bar and fix