An important point is that screen height, not width, should be the basis of your choices. The following may be obvious but I present it anyway.

Your images look the largest if you make the image as high (tall) as possible in the given room.

People appear at exactly the same size in a 16:9 image and in a 2.35 image if the screens are the same height. The difference is how much stuff you see off to the sides. Which is more important to the client -- how huge that thing in the middle is, or whether you can see the extras way out on the edges?

BUT if you go from a 16:9 of any width to a 2.35 of the same width, everything on the screen will be shorter in height, that is to say, smaller.

AND if you use diagonal to talk about the differences between 16:9 and 2.35 images, the width thing comes into play, since the width of the image is a factor in calculating the diagonal measurement.