To the best of my knowledge, the concept is that by setting no width/height values, it will maximize the window in both dimensions, not that this is always the outcome and height/width are affected by many things that may be going on in the browser. Modal windows are particularly difficult for me and in my early development, I found that scroll bars did not display on some browsers such as Safari, though I haven't tested that in a while. I think you just have to do a lot of testing in different browsers and screen resolutions and make good use of responsiveness screen settings.
