Is there an existing fan you're replacing? If a new install, you need to connect it to ducting that leads to outdoors, not just into the attic.

An alternative with more size flexibility would be to put in a small HVAC grill that runs through the ceiling drywall, to which you snugly connect flex duct on the back side, that then runs to an inline fan (like what Fantech makes) mounted in the attic. Then connect flex duct to the fan's output to run to where the air exits the attic and you're done. Remember to include a damper in the line.

Search for "Valterra Heating and A/C vent register" to see an example. Or get an HVAC ceiling grill with a boot behind it to mount. With a Fantech inline fan, you'll not hear much fan noise but a small grill and high air flow will produce air flow noise. Maybe a long and narrow one with a larger surface area would work?

Can you mount an exhaust fan on an exterior wall?