Use this guide to estimate what size cake you might need. This guide applies to standard round or square cakes. Carved cakes or different shapes will provide a different number of portions. Bear in mind that the design you choose may govern what shape of cake you have. My standard sponge cakes are approximately 5"-6" tall and four layers deep, so even a smaller cake may be big enough for your requirements.
To maximise the number of portions your cake will provide use the above method to cut it. A hot knife will also help with cutting.