I used 6" flex duct myself for a fan that I recently put it.
(I used the insulated stuff too)

I vented it to the soffit and put on a auto opening
grill to vent the bathroom air to the outside.

Nothing like having a 200 cfm van venting the bathroom.
No more smells, no fogging the mirrors nothing.
Just sucks the hell out of the heat in the house

Didnt want to go with a through room install. My feelings
are the less holes in my roof, the better off I am.

Tom