For me I like to use Earlene's serving size chart as the sizes are bigger. Using the Wilton cake serving size, a single layer 9 x 13 sheet is 12 serving at 2 x 2 x 2 inch servings. To get the 28 servings you'd need a double layer cake 4 inches in height.
You can figure this out by yourself if you want the cake servings to be bigger or smaller han existing charts by doing the following:
divide the area of the cake by the area of the serving. (to find the area, multiply width x height x depth).
For example: a 9x13 sheet cake with 2 layers will be an area = 234 inches
If the serving size 234/8 = 29.25 So your cake will serve about 27 to 30 people, depending on how the odd pieces are cut ( 9 x 13 has odd numbers left to make 1 inch space pieces of cake servings).
I'd rather error on having too much cake myself. Since you know this formula, rather than baking a totally large size, bake a 9x9 and tack it on to the end of the other if you want more. I do this all the time as it wastes less cake.
For me I like to use Earlene's serving size chart as the sizes are bigger. Using the Wilton cake serving size, a single layer 9 x 13 sheet is 12 serving at 2 x 2 x 2 inch servings. To get the 28 servings you'd need a double layer cake 4 inches in height.
I think you have a math error, in there. To get only 12 pieces, you're cutting the cake in 3 rows by 4 columns ..... 3" x 3".
I'm going to round the size to 8x12 just to allow for shrinkage and for easy math.
9x13, when cut into 2" pieces, mean the cake will be cut in 4 rows (9 divided by 2" = about 4) and in 6 columns (13 divided by 2" = about 6). 4x6 = 24 pieces.
A 2-layer, when cut into 1x2x4, means the cake will be cut into approximately 8 rows by 6 columns = 48 pieces.
I cut this size at all the family parties, and it really is a good size piece of cake. Perfect for little kids ... not too big and eliminates wasted cake. The adults are also happy with the size, especially if there is ice cream or other foods available. Trust me .. .I ahve my "mom-eye" on everyone and getting the verbal and non-verbal feedback on this! I also watch how other people cut the cake ... amazingly, other poeple tend to cut in a 2x2" square and in approx a 1x2x4 (although most people hand me the knofe on 2-layer cakes!)
indydebi - i am also starting to get requests for paid cakes, so I am somewhat new to this serving size dilema also. What is your serving size? Do you have a wedding serving and party serving? And does the heighth of the cake make a difference - well, I know it does, but what are YOUR serving sizes for for the different heighths? This obviously plays a role in pricing, and I am trying to figure that out too. It just took one cake for my boyfriends' supervisor at work, and I already have 4 orders. Any info would really, really help!
i always do 11 x 15 for that many people. i figure a 9 x 13 will only feed up to 20...but I like cake and like to see people have some left over after a party.
indydebi - i am also starting to get requests for paid cakes, so I am somewhat new to this serving size dilema also. What is your serving size? Do you have a wedding serving and party serving? And does the heighth of the cake make a difference - well, I know it does, but what are YOUR serving sizes for for the different heighths? This obviously plays a role in pricing, and I am trying to figure that out too. It just took one cake for my boyfriends' supervisor at work, and I already have 4 orders. Any info would really, really help!
I would suggest that you pick what you are going to call a serving size and stick to that. I do not do party servings. They are welcome to cut it any size they want, but they are paying for the number of standard servings the cake is designed to serve.
If you DO do party servings, it becomes a matter of semantics anyway, because a party size (1.5x2x4) is 50% bigger than a 1x2x4, so the price per serving should be 50% higher ($4.50/serving instead of $3.00/serving) and the price of the cake comes out the same.
Run the numbers:
12x18, 2 layer, when cut in 1x2x4 = 108 servings x $3/serving = $324.
12X18, 2-layer, when cut in 1.5x2x4 = 72 servings x $4.50/serving = $324. It's just semantics.
The problem is when folks charge the same for a party serving as they do for a standard serving. When I go to McDonald's, if I get a large fry that has more fries in it, I pay more for it.
Here's my KFC analogy: http://forum.cakecentral.com/cake-decorating-ftopict-597044-kfc.html (there's a typo in the numbers, which I correct in a later post on that thread). Memorize it and use it often! 
Here's how to cut a 2-layer cake: http://www.cateritsimple.com/id10.html
Here's what a 1x2x4 piece of cake looks like: http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1156785
I don't get the whole party serving/wedding serving thing. I know the sizes are based off the fact that at a wedding the cake is not always the only dessert, hence the smaller serving. But the wedding serving is really perfect for both events. I think you really need to cut a piece a cake the Wilton's size and see. Not to mention Wilton's is not the only one who cuts cakes that size. Go look at a piece of cake in a Martha Stewert's Weddings mag. The pictures that they show of pieces of cake are 1x2x4 size servings.
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