Here you go. http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm This chart (which is also in every Wilton yearbook) has a ton of info you need. Including servings per cake pan, how much batter and icing, etc.
http://www.wilton.com/cakes/tiered-cakes/ This one has lots of info on tiered cakes. If this is your first, keep it simple. Just use two layers of cake with a layer of icing in between for the filling. Then ice the outside. You want to end up with a cake that is around 3 1/2 to 4 inches or so tall including the icing. Nothing magic about this, nor do you need to be precise, that is just what you see in most pictures. And what the cake servings are based on.
Lastly I highly recommend you spend some time on youtube with seriouscakes and tonedna1. They both have a bunch of fantastic videos that are good quality and cover a LOT of topics, including stacking and smoothing your buttercream.
As far as round or square, it really is just a matter of number of servings you need and what look you prefer. No right or wrong way, just preference to your eye.