7 feet , while adquate for most of us, will look funny, and may be negatively looked upon at resale time. Are you not able to recess the fan/light in between existing ceiling joists?
A fan/light directly over a shower or tub must be specifically UL listed for that application. Most that you find in the stores are NOT, but listed units are available . Ask to see the Broan or Panasonic catalog.