You use the wieght of the eggs as a basis, as they're the thing you can't change.
But it depends how big you want it, and I use this recipe mainly for little cupcakes, it's been a while since i did it with a big cake, but it still works. using 3 eggs gives 8 reasonable slices. And the oven should be pre heated to 180*c.
weigh the eggs then whatever they come to, weigh out the same of margerine, sugar and flour. Cream the butter and sugar together for a good 5 minutes, with a wooden spoon, then whisk in the eggs until there are no lumps. Fold in the flour and place the mixture into a greased/ baking paper lined tinned.