How tall should a wedding cake be that has 12-9-6 teirs, how tall should each teir be?

It depends on how thick your layers of baked cake are....and how many layers are in each tier.

There's no "right" answer.

Some people do cakes with double layers inside, some do three, some do four....

Is the cake to be the main dessert for the evening? If yes, you might want the slices to be triple-layer inside so it looks extra special on the plates. If no, you can keep the slices a little smaller at double layers.

How big are the slices you are quoting the bride? 1" x 2" x the height? 2" x 2" x the height? 1" x 3" x the height?

My standard 6"/9"/12" tiers are triple-layers of cake inside each tier. Each tier is roughly 5"-5.5" tall, or even taller if we add fondant icing on the exterior.

Mind you, I have other friends that have quadruple-layers inside as their "standard" slice size.

Depends on you, how much cake they really need, what course they are serving it at and what you've decided to charge per slice, really.

The short answer is, 4 inches tall.

That's the standard in the biz.

If you want to design something shorter or taller for the purpose of design, go for it.