How do I make my cupcakes and muffins a consistent size?

Though nothing is perfect, nor is that necessary, to make professional looking your cupcakes and muffins, use an ice-cream scooper for even sizes.

Similarly, for drop cookies, you can buy a melon ball scooper, or whatever size you want, for maximum evenness. You can use a measuring spoon, but honestly, is easier with the mini scooper.

For cakes: To make even layers, use a ladle to transfer batter to pans (as opposed to pouring a bit in one pan and then some in another, etc.), and that's the best I've come up with.