Can anyone tell me the size of a dessert size piece of cake as opposed to a coffee size portion and any ideas as to what size cakes i will need to make for a round or square two tier to serve 100 in dessert size portions????? would be greatful for anyones help Kelly

Here you go:

A dessert portion is roughly 2.5 x 5 x 9 cm high, while a coffee portion is half the size of a dessert portion.

AU sites with more info:

http://www.sweetart.com.au/faq.html

http://www.planetcake.com.au/couturecakes/faq.php

HTH