Anyia, it depends what type it is, if its like mine and just recirulates the air (through a carbon filter) then there should be a gap big enough for you to get your hand in to clean the ceiling when you forget to change the filter.
I wasn't joking, about 6 inches minimum, if yours has a vent pipe then it will need to be low enough to get the vent pipe up and out through your chosen route.
vent pipe being like tumble dryers have (sometimes. You know the one most people poke through the window)