If you have a use case, where you want to compute dialog/wizard size based upon the system client area(system resolution),then the following is the general approach.
Rectangle clientArea = Display.getDefault().getClientArea();
int x_hint = (int) (0.7 * clientArea.width); //% based on the required size
int y_hint = (int) (0.7 * clientArea.height));//% based on the required size
GridDataFactory.fillDefaults().grab(true, true).hint(x_hint, y_hint).applyTo(this);
The problem comes with above approach is, if your primary monitor is connected with another monitor in extended mode display, then the dialog/wizard may stretch to the extended monitor also, considering system calculates the both monitors width as a display area.
To avoid this kind of issues, you can get primary monitor display area and compute based on that.
Monitor primaryMonitor = Display.getDefault().getPrimaryMonitor();
Rectangle clientArea = primaryMonitor.getClientArea(); //Now compute based on this.