Open ConfigurationWindowController.xib, select the window, and switch to attributes inspector. Uncheck Resize to disable resizing for the window. Also uncheck Visible At launch.
That last step is an important step to remember when working with sheets: if you do not uncheck Visible At Launch, the window will appear before it is presented as a sheet, which will make your application look buggy and broken.
Figure 24.5 Inspecting new window
Drag out onto the window two labels, two buttons, a color well, a text field, and a stepper. Arrange everything as in Figure 24.6.