I use a 4" duct fan and the vent looks just like your dryer vent on the outside of the house. It does not let air in. It is not air prrof but it is not like have a box fan in the window. Some people use bathroom exhaust fans for this.
My previous sump was in a 4'x20' room and it got very humid and made the tank temp hard to keep stable. I used the vent fan(cost $20 or so) and ran 4" dryer vent mixed with some PVC
to vent it outside. I had to run it about 30 ft as well. Worked very well for as simple as it was. You caould always put a humidistat on it so it only comes on at certain humidity levels.
Also keep in mind that chiller create heat. If your sump and chiller are in the same enclosed room, you could cool the water only to heat the room thus making it harder to cool the water.
The bigger the sump room the better of course. Is your tank and sump going to be in a different rooms? I moved my sump to the basement so it acts like a chiller
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,