How to make full width image slideshows
I can see the image slideshows in full width for 1 sec but then images
  go back to original sizes.


sounds like there is another css that is loaded after that is causing them to shrink. (perhaps it's in the views slide show css file). To prevent anything that is loaded after from overriding your css try adding the !important tag

so try this:

.views-slideshow-cycle-main-frame-row-item .field-content img {    
    min-height: 500px !important;
    width: 100% !important;
}