Start at the bathroom door with a tape and a notepad. Measure the floor length and width and the ceiling height. The room's volume sets the airflow the fan has to move, so that figure drives every later choice.
Read the label on the housing
The fan carries a label, usually printed on the housing above the grille or on the motor. It lists an airflow rating and a sound rating. Copy both before anything else. Compare the room against the sizing chart in the maker's installation sheet, not against a rule of thumb you heard from a neighbor. If the chart and the label seem to disagree, ask the installer which one governs the model in front of you.
Measure the duct, not only the fan
A fan moves less air through a long duct with bends than through a short straight one. Ask the installer for the total run in feet, the count of elbows, and the duct diameter. Flexible duct that sags or gets crushed in an attic cuts airflow more than most people expect. The discharge must go outdoors through a hood, never into an attic, a soffit, or a crawl space. Look for a backdraft damper at the hood so outside air does not return into the bath when the fan is off.
Let the household set the controls
A bath used by several people in short bursts needs a different control from a guest bath used now and then. Decide between a wall switch alone, a timer that keeps the fan running after the light goes off, and a humidity sensor that starts the fan when moisture rises. Ask whether the timer or sensor can be adjusted on the unit, and who will change the setting later. A sensor left on its factory setting may run too little or too much for the way the bath is actually used.
Give the room a way to breathe
A tight bath with a closed door and a fan pulling air out needs a path for replacement air. Check the gap under the door and whether a transfer grille exists. Without that path, the fan works harder and pulls air from wherever it can reach, often the attic or a flue or chimney space you would rather it not draw from.
Check the ceiling and the access
Note the joist spacing and whether insulation touches the housing. Where insulation sits against the housing, choose a housing rated for insulation contact. Confirm the grille comes off for cleaning without special tools, and that the motor can be reached for service without cutting the ceiling.
Write the plan into the house record
Keep the room dimensions, the airflow and sound ratings from the label, the duct run and elbow count, the hood location, the control type, and the install record together in a single place. Name who cleans the grille and the filter, and who checks the outdoor hood each season. That record is what the next person needs to keep the fan moving air.