Portions and combinations
If you have chosen your flavours combination but at this point you are not sure how you would like your cake to look like, take a look at the most popular and classic cake combinations. It will give you a rough idea of how you could divide the cake.
Take in to consideration that you also have the possibility to use dummy tiers which will make your cake bigger than you actually need. These faux tiers have the cost of approximately half of what the real tier would cost.