The size of your wedding cake depends on how you plan to serve the cake.
If your wedding cake is your main desert and the venue is going to cut and serve it, than you only need a cake large enough to accommodate all your guests. Consider one serving per guest. A 200 guest list would require a cake that could be cut and served into 200 pieces.
If your cake is part of a sweet table, only get a cake to serve half of your guest list. There are to many other options on the sweet table to choose from. Cakes tend to loose out.
If your wedding cake is purely for decoration purposes than the size of the cake will depend on what your theme is and what you are trying to achieve.
Choose your cake bakery very carefully. You can not repeat the process. Seek recommendation from friends. Surf the web for information about the baker. Check out the bakers web site to see the photo gallery.