i know about the charts for serving size, but this is my ?: if a cake is taller than the average 4inches (sat one tier is 3 - 2inch cakes, making it 6 inches high) is the serving size the same? will most people still cut it the same?
Unless you want to put a board in the middle and make to 3in tall sections then yes I would consider a 1x2x4 or a 1x2x6 a serving size. Most people are probably not going to want to deal with cutting it in half.
One thing to note though, as far as pricing goes, I would price a 6in tall tier more than a 4in tall tier, while they are getting the same number of servings, they will be getting more cake with the taller tier.
I'd also consider if a 6in tall tier would fit on the dessert plates being used.