you can use a basic all-in-one sponge mix recipe and test what quantity works best.
I presume it is a round tin, so depending on depth I'd try a 3 egg mix to start with.
the formula is dead simple - for every egg (medium size) you take 2 ounces of sugar, butter and self - raising flour each. then just multiply quantities as you wish.
try 3 medium eggs, 6 oz sugar, 6 oz softened butter and 6 oz of self raising flour and 5ml vanilla extract. mix with spoon until combined (or with a machine), then pour into tin that's been greased and floured.
bake in pre-heated oven on gas mark 4 for 35-40 mins, middle shelf.
very important - write down what you did. if cake's ok - fantastic.
if there's a problem just adjust quantity or temperature or baking time until you get it right.