Bug 33550 [Dialogs] Dialog should support remembering size and position
There are several dialogs that remember its size and position. Since restore the size and considering screen size and font changes isn't trivial JFace's Dialog class should support remembering and restoring size and position. These would realy ease the task for subclasses. May be you can steel the code from OpenTypeSelectionDialog.
bundles/org.eclipse.jface/src/org/eclipse/jface/dialogs/Dialog.java
return null;
        GC gc = new GC(control);
        int maxWidth = control.getBounds().width - 5;
        if (gc.textExtent(textValue).x < maxWidth) {
            gc.dispose();
            return textValue;
        }
        int length = textValue.length();
        int pivot = length / 2;
        int start = pivot;
        int end = pivot + 1;
        while (start >= 0 && end < length) {
            String s1 = textValue.substring(0, start);
            String s2 = textValue.substring(end, length);
            String s = s1 + ELLIPSIS + s2;
            int l = gc.textExtent(s).x;
            if (l < maxWidth) {
                gc.dispose();
                return s;
            }
            start--;
            end++;
        }
        gc.dispose();
        return textValue;
    }
    /**
     * Create a default instance of the blocked handler which does not do
     * anything.
     */