How long is the duct, how many bends, what's the duct made of?
These are things that go into determining what the theoretical amount of air a fan will push through that duct. Someone will be along shortly with a pretty good rule of thumb though and help you out.
Most fans come with a chart showing performance over duct run length. As OOC mentioned, bends add to the theoretical length. Go for the straightest, shortest run. Use metal instead of flex, if possible.
My limited experience -- we needed a 100 cfm fan for the recommended air changes per hour. Couldn't find a 100 cfm, so we got the next model up. Duct run is about 4 feet to the roof soffit, where it goes 90 degrees down and through a basic plastic vent grille (use one that has more open space than material, or that will limit the fan's performance, too).
Make sure there is a way for air to get into the bathroom when the fan is on and the door is closed. Space under a door works well, ours is about an inch over wood/tile.