The Panasonic site has a fan sizing tool that would probably tell you that an 80 cfm would be fine. I talked to a guy who sells them, and he said the variables that would indicate upgrading to the 110 cfm model would be the number of turns the venting takes, the distance for the venting, and whether the room has lots of hard surfaces that would tend to generate more condensation (e.g. tile, solid surface counters, lots of glass in shower area). I think I'll get the 110 for my 8x8 room, just to be safe, since I am right at the top of the square footage range for the 80cfm.

We have the 110 in our upstairs bathrooms - very quiet operation.

Make sure you have enough 'intake' to supply the fan, especially if you go for the higher volume one.

You need a decent gap under the door to allow as much air in quickly enough for it to be vented out.

I put Panasonic fans in both my bath remodels and love them. 5 x 8 room with 8 foot ceilings only needs 35 or so CFM (5 x 8 = 32 x 1.1 = 35.2) so I'd stick with the 50 CFM models.

Keep your duct runs short and straight and made of proper metal ductwork screwed and taped (foil tape). No flexible plastic or metal, especially if you want to keep it quiet.

Here is a link that might be useful: Panasonic fan sizing

What matter most is if your vent is small or large, long or short, flat-wall or corrugated . It's the tube that carries exhaust air outside.

Also important is the available air (e.g. door gaps) to allow incoming air to replace the air that gets "exhausted".

There is nothing significant between 110 and 80 cfm, for all intents and purposes. Quiet operation is your main goal. The noise one might hear if the fan were turned on prior to installation is not the noise made after installation. All the vent "tube" factors listed above make a difference in terms of whether or not the fan has to push very hard against a lot of air resistance or if it only pushes reasonably hard. Some fans make a lot more noise when conditions are sub-optimal, some don't make a lot more noise. Manufacturers don't compare their products against competitors and publish results, but they could do this if a hundred people asked for it. Real world installations are the thing to compare, not showroom installations where the vent tube only runs a few feet and then blows air back into the same room.

Thank you very much for all the responses above, which are very helpful.

The bathroom is measured more accurately as 49 sq ft. There is a square skylight (22x22) on flat roof. I would like to have it changed to the Low-e double pane one. But usually the skylight would be left open.

There is gap under the door. I don't think the duct would be long. There are granite vanity top and tub tile walls.

According to the Panasonic fan size link, it looks like the 80CFM should be enough. Am I right?

Thank you very much.