Bathroom Fan Upgrade Needed

I currently have a bathroom in which I also have my closet. There is a bifold sliding door to the closet separating the it from the bathroom(though it stays open almost always). The two rooms together consist of 108sqft. I want to keep the humidity out of the closet which is why I want to upgrade my vent fan.

I live in a lennar home and have the cheap builder fan they installed when they built the house that is attached to a 4" vent (broan 688). Any suggestions on the CFM I need? Also will the 4"-6" vent adapter defeat the purpose of a larger vent fan?

Thanks!

 

Ballpark figure is that you need a fan that moves around 100cfm. The really cheap fans move maybe 50.

You can upgrade the fan assuming you have access to replace and that will help.

Want really good airflow, look into the inline fans.

Put one in our bathroom when remodeled it's around 300 cfm and uses a 6" duct. It will clear a room full of steam in 20 seconds!

 

My vent is a 4" do you think it will affect the CFM airflow by using a 4" to 6" adapter?