How much air do I need?
This depends on your building codes and any openable windows. If you have windows that open, this usually meets the legal requirements, so you are only interested in removing moisture. If you don't have windows, check your code as to the minimum. The NZ and Australian codes say 25l/s per bath, shower or toilet in the room – this means 50l/s for a shower and toilet combo. This is a good starting point.
The other way of doing it is based on the room volume and is a useful double check. Measure the room volume (say 20 m^3) and the airflow of the fan (say 120 m^3/ hour). The number of times the room air is changed is 120/20 = 6 times per hour, which is about right in many cases. Choosing a fan that is to big means more noise and possibly drafts, but it is easier to fix than one that is to small, especially with undersized ducting as well.
Remember that air can't get out unless it can get in. Undercut the bottom of the door by 15mm or so in order that the air can get into the room to replace what is taken out. You may even need a grille if you are taking a lot of air out.
An obvious point, but often overlooked when designing a house, is how to ventilate the internal rooms. Some building authorities still allow you to dump the air into the ceiling space, but this is gradually changing (and not before time). The moist air from your bathroom promotes rot, destroys the insulation and can warp your ceiling panels. In NZ the air has to be exhausted outside, and I urge you do the same, whether or not it is compulsory. This could go out through the roof via a pipe with an upside down U-bend or cowl on the top, or through the soffit or sidewall using a grille.
The exhaust point should not be close to a window or door where the air might be drawn inside again, nor should it blow over people passing by. Again, the building codes may have some minimum distances here. The Australian standard calls for 6m separataion from any inlets. This is not always possible in houses, so use your common sense. Also, check out Codecheck.com
Run on timers
I recommend that everyone use a run-on timer. This is a little box-o-tricks, often built into the fan, which lets the fan continue running for a set time after the power supply (eg the light switch) has been turned off. It should run long enough that the air in the room is changed at least once. The reason for doing this is that you finish your shower, dry off and rush out of the house to work, leaving the room closed up. Unless the air in the room is extracted, most of it will still be very moist, as the fan won't have had a chance to clear the air before it was switched off.
The aim of locating the fan is to get the air to flow across the room, without taking a shortcut and leaving an area unventilated.. This means if there is a window, the air shouldn't come in the window then straight to the fan without passing across the room first. It is really a matter of common sense. Put the intake as close to the problem as possible, which normally means over the top of the shower stall.