Traditionally tiered wedding cakes aren't cut into large portions. The cake tends to be served after the wedding breakfast, during the evening reception and is not usually intended to be a dessert.
Sponge cake is typically divided into 2" x 1" portions. Fruit cake is much more rich and is typically divided into 1" x 1" portions. You don't need to worry about calculating how large your cake needs to be to ensure everyone gets a piece; just let us know how many guests you're catering for and we'll work it all out for you.
If you intend to serve your cake as a dessert then a tower of cupcakes works very well. It can include a top tier for the purposes of 'cutting the cake', and a large cupcake for each guest. If you would like to serve your tiered cake as a dessert just let us know and we'll scale up the size accordingly.