If it's for a wedding then I guess you serve one piece per person, but how do you know how many pieces a cake cuts into.
Crumbs Celebration Cakes from Koo Wee Rup in Gippsland has a handy-dandy portion size chart on their website. This tells you how big a "coffee portion" is and how big a "dessert portion" is, as well as how many of each you get from a particular size cake (of course if it's for teenage boys then it has to be a bloomin' big cake with big portions!).
They also teach classes in piping and cake-making/decorating. Here's a couple of piccies from their site (there are heaps more cakes on here):