Go with a 5" diameter port if it's going to be 1.5" long.As an aside, these ports are pretty big so maybe some people would want to use a wire mesh to keep the pets out. If so, I'd go with something with very large holes rather than a thin mesh. That should keep it from impeding airflow.
I don't use such a mech in my tower two's, but I have them pushed back within a few inches of the wall.