Which is correct for instance size

[lokiys]

instance.SetWindowSize(project.Profile.AvailScreenWidth, project.Profile.AvailScreenHeight);
instance.SetWindowSize(project.Profile.ScreenSizeWidth, project.Profile.ScreenSizeHeight);

This available returns minus x pixels, for height, meaning it's considering taskbar if i understand it right, But then which should i use real screen resolution or this available in that method, to look natural?

There are a couple results
Screen Resolution Available: 1536x816; Screen Real: 1536x864
Screen Resolution Available: 1366x738; Screen Real: 1366x768