Scaling down cake ingredients
This is for owners of kitchen scales: I find cups too messy and imprecise - sorry! If you need to convert into cups or from cups, use the excellent cooking converter.
Start with eggs
The simple way is to see how many eggs your original recipe calls for - if indeed it contains eggs; but most cakes do. If it doesn't, just whip up a calculator and slash your amounts by a third or multiply them as required.
If the original recipe wants 4 eggs and you want slightly smaller cake, chalk up 3 eggs and then calculate ¾ or 0.75 of everything else. If the opposite is desired as you want a really big cake, do a reverse exercise. Maths does come in useful sometimes. So does a calculator.