When doing a birthday cake at work i use 12 eggs and 24 oz of Flour and 24oz of Marg and 24 oz Caster Sugar. But you don't really need one that size, so for a small one, i'd got with 3oz of everything to 1 egg. The size you probably want it like 9 oz of Self Raising Flour 9 oz of Caster Sugar and 9oz of Marg and 3 eggs mix all together and then greese two tins and pour the mixture equaly into both tins and then into a pre heated oven at 180 for about 40 mins.