About size: no matter how big is your tank, a big fish will always make it look small.
Another factor that influences our perspection of size is behaviour. A fish that is all over the place, that within 3 ou 4 seconds gets from one side of the tank to the other, will always make your tank look small - and worse than small, it will make your tank look like a prison.
So, in my opinion, small fish that don't swim contantly randomly through the tank, that hang mostly in a specific area of the tank are the best choices. They also allow you to take to the extreme this option of taking in account the fish's appearance in the tanks layout.
Lets take a look at clownfish for example.
A clownfish in a bubbletip anemone will look good because what you get is a picture of warm colors. The darker and brighter areas of the clownfish provide a rich contrast.
A clownfish in a blue anemone provides a stronger contrast and it still looks great, but more dramatic:
If your tank is a mainly a coral tank and you want to call attention to the corals, then the fish should not have contrasting colors and or white or black.
If you want to call attention to the fish, then the coral should have mainly contrasting colors.
Once again, the colors of the fish can be in harmony among themselves. For example, you can have one or two very special fish, and choose the other fish so that their colors provide a "background" for your main fish to shine. Ideally they will have contrasting colors with low saturation - less bright.
If your "show" fish is mainly bright yellow or orange, a group of blue chromis will look great. A red fish will look great with a group of green chromis. Contrast is size and shape can also be taken in account.
I've been talking about fish, but the same goes for other animals.
