Installation in existing ceiling, not new construction. Instructions do not include a template or mention how big a hole to cut. I can only assume hole is as wide as bent edges on open end of housing. That is the only way housing tabs can contact bottom of joist.
Thank you for your question on the model QT90E exhaust fan. If you have access from above (an attic or crawl space) you would want to cut the hole the size of the bottom outside flanges of the unit housing. You could then drop it down through the hole and mount it so the bottom flange is even with the finished side of the ceiling. If you do not have access from above, you will need to cut the hole larger than would normally be needed and patch the ceiling after it is installed.
Thank you for your question on the Broan QT90E ventilation fan. It is recommended, where possible, to use rigid duct. It has less resistance to air flow and allows the fan to operate much more efficiently. If flexible duct is used, be sure the duct is as straight as possible.
Thank you for your question on the model QT90E exhaust fan. The QT90E can handle up to 18 feet of straight, four inch, rigid duct work. You would need to reduce that distance by four feet for every elbow.