This is a question we get asked a lot, from both wedding and occasion cake customers. The general rule of etiquette (but don't quote me on this) is to have a piece available for each guest. Now, we all know that there is that weird Aunt who is always on a diet and will never touch a piece of cake, or that picky eater who doesn't like the flavor offered, but on the other hand, there are those guests who will have two pieces, or that kid who drops their piece and wants a fresh one, so they tend to balance out.
Another thing to consider when determining how big the cake needs to be is what type of food is being served with the cake. At many birthday parties, there are a few appetizers, but the cake is the main event. This tends to require a little more cake as the pieces tend to be larger. At most weddings, there is a full meal served, with the cake being dessert, which is why the pieces are usually cut a little smaller.
The last thing to consider is the length of the event. If your event has a definite end time, then people are less likely to have more than 1 piece of cake, but if your party guests tend to linger for a few hours, then they are more likely to nosh on an extra serving as time wears on. For a wedding, if you are going to have the cake out on a table while your guests are dancing, you can rest assured that after an hour or two of dancing, guests will start to grab another piece to keep up their energy.
Should you find yourself with a little extra cake leftover, you can relax knowing that the cake tastes just as good as the next day!