Here's a link to Wilton's wedding and party serving size charts (for 2 and 3 inch pans):
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
Serving size template (to help visualize cake portions):
http://www.cakecentral.com/cake-decorating-photos_display_121_-88648.html
Threads on Wilton serving sizes and others (also other serving charts):
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
http://www.cakecentral.com/cake-decorating-ftopict-50643-serving.html+size
http://www.cakecentral.com/cake-decorating-ftopict-49110-serving.html+size
http://www.cakecentral.com/cake-decorating-ftopict-47742-serving.html+size
http://www.cakecentral.com/cake-decorating-ftopict-42361-serving.html+size
Serving size and pricing guide:
http://www.cakecentral.com/cake-decorating-ftopict-43641-serving.html+size
HTH
Locally, a 12x18 half sheet single layer will serve 54 (2"tall x 2"deep x 2"wide)
A 12x18 double layer half sheet will serve 108 (4"tall x 2"deep x 1"wide)
In most cases those sizes will be enough cake for most people, especially after dinner, and more especially if served with ice cream.
If you or your client requires more cake, make more cake and cut bigger pieces.
Well you need to decide how big you want your slices to be. a single layer 2x2x2 for a 12x18 =(6x9)=54 or for a double layer I would do maybe 1x3x4=(12x6)=72. or 1x2x4= (12x9) =108 pieces. Just have to do some math!
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