Below is the approximate portion guide for single tiers, but it will give you a guide to tier size required to feed your guests.
After the wedding breakfast and possibly a buffet, your guests will not require a large piece of cake. My recommendation is that you cater for 80% of guests wanting a piece of cake. I also suggest a portion 1" x1"; they are getting 3 layers of sponge with filling, working out about 5" deep. A nice size piece of cake after all that food! It will also ensure you are not left with a lot of wastage.
The portion guide below suggests on a 2" x 1" piece of cake. Three layers of sponge with filling, working out at 5" deep. A piece of cake that is perfect after a dinner as dessert.
Obviously you can cut the slices in to large pieces, more coffee style, but you will not obtain as many portions from each tier.