如何制作全屏div，并防止内容更改大小？

for my web application, i would like the main div to be full screen (both width and height = 100%), and regardless of content, i want it to stay at that size. that means, if there are not much content, it shouldn't shrink, and if there are too much content, it shouldn't push this main div.for my web application, i would like the main d