The table below gives a rough guide to the number of portions you can cut from square or round cakes of various sizes.
Rich fruit cakes are traditionally cut into pieces measuring 1 inch x 1 inch, whilst sponge cake portions are usually larger, about 2 inch x 1inch. This means a sponge cake cuts into fewer portions than a similar sized fruit cake ~ the first two columns in the table below below show number of portions for a fruit cake, and the last two are for sponge cakes.
Shapes such as ovals, hearts and hexagons will give roughly the same number of portions as a round cake of the same size. Bear in mind that square cakes are easier to cut into rows of neat and tidy portions than round cakes!