What is the suggested/ideal size for the jumbotron image?
Speaking of image size…
what is the suggested/ideal image size for the testimonial featured image?
There is not ideal size for the jumbotron as it takes the height of the image and this goes different for every website.
In the demo, the header image is 2000x1250px, so you should definitely try a wide landscape photo.
For the testimonials, the preview is 127x127px