you have a 40 cubic ft tent , you have a 100 cfm fan CFM definition = cubic ft per min. SO your fan is almost 3 x bigger then what you need ,So add all the goodies you got in the package deal and Yes the fan is sized correctly, And you think its to small , But then you say if you leave the tent open the temp drops , So there is your answer. think of your tent as a small sandwitch size plastic bag if put your moulth over the opening and suck in what happens to the air , And what happens to the bag, ok Now put a hole in the bag about the size of a dime , what happens, Ok now cut the whole bottom out of the bag, So you have the dime situation goin on SO we need to figure out what size fresh air intake you need to have that fan exchage the rate of air that it is rated for 100 cfm , you can go with a 6 inch round supply fresh air ,but you are boarder line with 6 inch I would use 8 iches of round for intake ................