The image for the wide banner can be as wide as you want it to be, and the short is 1140px in width. Your height can be anything you want as the height of the banner container area will self-adjust to it.
As for the tutorial, I didn’t realize I did it on Dynamic Widgets…it’s a very powerful plugin but does have a learning curve to it. After I did that theme, I came across the Display Widgets one which I found to work easier for most and simple to use. Both plugins can achieve the results you want, but I would say for keeping it simple, use the Display Widgets plugin.