First about how to get them sized: I've been doing a back search and it seems logical and recommended to kind of cookie cutter out pieces from a larger cake for mini cakes. It probably gives you moister pieces and more "straight/flat" tops
Question is though: where do you find 4", 3", and 2" circles?
Next, how tall do you make them? If I have 4", 3" and 2" tiers, how tall should each be? Should it get shorter as it gets smaller? Or is it 2", 2" 2" tall of cake (will get taller with filling? I can't imagine having it start out as a 4" cake at 3" tall, but maybe it should?
Or should it be 2", 2", 1.5"??? Am I overthinking it here?
I have one of those little 2 tiered mini cake pans and the whole thing with both tiers is like 3 inches tall, I made one with a 3" round cake underneath and the whole thing was the height of a can of soda. I think the minis are more around 1 1/2 inches tall per layer.....but that depends on the look you're going for too!
I hope this helps ... if not maybe you can do a similar search for something more to the size you are looking for ... I read about biscuit cutters being used for mini cakes in a cake book