How do you determine the size of the serving for a 6" tall tier.I know that a 4" tall tier is to be cut 1"x2"x4". They are getting 2" extra of cake in the 6" cake and I have determined how much to charge for that but I need to know the dimensions of the serving, would it be 1"x1"x6".? This is for a wedding cake that I need to price out today. HELP!

It depends on your construction.... if you'll use a board in the middle for 1x2x3 servings, or slice through the entire 6" tall tier.

This calculator by volume should help:
http://shinymetalobjects.net/cake/calculator/cakulator.cgi?option=3&option=Select+Cakulator

According to Wilton which is kind of an industry guideline:
Serving amounts are based on wedding-sized portions of approximately 1 x 2 in. Cakes from 3 to 6 in. high, baked in the same size pan, would yield the same number of servings because they follow the same pattern of cutting.
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Uness the person cutting the cake was given special instructions on how to cut the cake since it's taller or you did as CWR suggested which is put a cake board between them, you wouldn't get more servings.

If you do put a cake board between them and the cake server uses the standard cutting format your slices would be 1X2X3 which is only 6 cubic inches where the standard wedding cake serving is 8 cubic inches.

The above is commentary for how many servings because it's specific to the person who is cutting the cake.

For pricing the cake, ingredient wise and cost wise a 6 inch cake that is 6 inches tall is 1.5 times as big as a 4 inch tall version of the same cake so for pricing you need to increase your price by 50%. For the # of actual servings, that's based on the cutting.