The first thing to figure out is how many people you need to serve. Our wedding cake needs to serve about 80 people, so I turned to trusty Wilton to tell me how many servings each cake size yields. Here's a handy dandy chart, but this is where I netted out:
I chose to go with four tiers, in diameter of 4" to 10", for a couple reasons: 1) I didn't want to buy or figure out where to store a 12" cake pan, and 2) I read somewhere that wider tiers actually make a cake look shorter, even if they are actually the same height. So, four tiers of 6" to 12" would be equal in height to four tiers of 4" to 10", but the former would actually look shorter than the latter, thanks to optical illusions (and magic).
In terms of height, this one I went back and forth on. I use 3" high pans, and I torte each pan to yield two layers of about 1" each because less pans = less dishes, and I feel like a ninja when I'm slicing cake in half. Knowing I wanted something more grand than just 2-layer tiers, I thought the easiest way to do this would be to just use two cake pans per tier, giving me four 1-inch layers for each tier. However, add in 1/4" of frosting between each layer and another 1/8" of fondant, and each tier would be a towering 5+" tall, which, when I tested it with a coconut cake, was A) intimidating, B) heavy, and C) would not fit in any cake box for traveling very comfortable. With four tiers, the cake would be nearly two feet tall stacked. So, conceding that I might have to waste cake (who am I kidding, I'll still eat the unused layers), I decided to go with three layers per tier.
I'm a big believer in the inside being just as beautiful as the outside. My dad is the main influence there: he's an avid DIYer on homeownership projects, and fixes vintage muscle cars as his hobby, and if the grout under the tiles isn't beautiful, or the engine under the hood isn't sparkling clean, then it's not a complete project.
Knowing I wanted to do both chocolate and vanilla (crowd pleaser on both ends), my first thought was to just alternate the flavors by tiers. My 10" tier would be vanilla, 8" would be chocolate, 6" would be vanilla, and 4" would be chocolate. After mapping out the servings though, I realized that would leave me with 50 vanilla servings and 32 chocolate servings -- an odd ratio. So then I thought maybe 10" and 4" to be vanilla, and 6" and 8" to be chocolate, which would give me 46 vanilla servings and 36 servings chocolate -- a closer, but still not even ratio.
To solve these uneven ratios of vanilla to chocolate, my second thought was to do a checkerboard cake. They're all the rage on the Internet, and I absolutely loved what came out of How to Cake It and Cookies, Cupcakes, and Cardio. Poor frosting and fondant application on my test cake notwithstanding, I didn't love the way it all melded together. Sure, it looked cool, but the delicate vanilla was overpowered by my/Amy's Bread's (amazing) chocolate recipe.
I finally landed on mixing the flavors within the tiers, but not within the layers, so everyone gets to taste both flavors, but they can still enjoy the vanilla and chocolate either individually or together. So, each tier is either chocolate/vanilla/chocolate layers, or vanilla/chocolate/vanilla.