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
protected void cancelPressed() {
        setReturnCode(CANCEL);
        close();
    }
    /**
     * Returns the number of pixels corresponding to the height of the given
     * number of characters.
     * <p>
     * This method may only be called after <code>initializeDialogUnits</code>
     * has been called.
     * </p>
     * <p>
     * Clients may call this framework method, but should not override it.
     * </p>
     * 
     * @param chars
     *            the number of characters
     * @return the number of pixels
     */
    protected int convertHeightInCharsToPixels(int chars) {
        
        if (fontMetrics == null)
            return 0;
        return convertHeightInCharsToPixels(fontMetrics, chars);
    }
    /**
     * Returns the number of pixels corresponding to the given number of
     * horizontal dialog units.
     * <p>
     * This method may only be called after <code>initializeDialogUnits</code>