I have several cake dummies I put out on display during a consultation. The cakes are common sizes... 3 tier all round cake, 4 tier all round cake, 5 tier square cake, 4 tier mix and match shapes cake. It makes it easy to say "see the 4 all round tier cake, that feeds 120 so that is the size of your cake."
I also have my porfolio of cakes I have made out for clients to look at. If they see a cake they like I can point to one of my dummies to show a comparision of size.