I am making a sheet cake for about 80-100 people. I have always made 1 9x13 white cake and 1 9x13 choc cake for a 1/2 sheet- Is this the correct thing to do? For a full sheet would I use 4 of the 9x13 cakes and piece together or should i just use the 15x20 sixze? Will this be enough for 80-100people. Thanks for help in advance.
"sheet" size seems to vary where you live. Two double-layer 9x13 would serve approx. 90. Here's a Wilton chart for party cakes - http://www.wilton.com/cake/cakeprep/baking/times/party_2inch.cfm
A 9x13x2" cake should serve 20 2x2x2" pieces.
Double that = 40 whether you make it 18x13x2 or 9x13x4.
To serve 80-100 you will need 4 or 5 9x13s no matter how you put them togetehr. If they are 4" tall then the 'slice' will be 1x2x4; if the cake is 2" tall the 'slice' is 2x2x2 -- each is the same amount of cake just cut differently.
BTW: I don't think I've ever heard of a 15x20 pan 
There are 11x15, 12x16 or 12x18 pans.
Hi and Welcome, Kimlee. 
CC acronyms:
http://www.cakecentral.com/cake-decorating-ftopict-2926.html
Here's the link to all four Wilton cake and serving charts so you can choose either the wedding size (1x2x4) or party size (1-1/2x2x4) yields:
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
(Includes both 2 and 3" deep pans.)
Have you considered using square pans:
12" would yield 96 party servings (layered cakes back to back).
16" would yield 80 party servings (single layers back to back).
Indydebi does sheet cake math:
(You decide on 2x2 or 2x3 slices.)
http://www.cakecentral.com/cake-decorating-ftopict-302303.html
Previous threads on sheet cake size terms:
http://www.cakecentral.com/cake-decorating-ftopict-234751.html
http://www.cakecentral.com/cake-decorating-ftopicp-3010773.html
Found a 15x21x1 pan which is described as 3/4 sheet:
http://www.ablekitchen.com/Sheet-Pans-3-4-Size-15-X-21-X-1-p/jr-10295.htm
Using this size layer cake would yield 70 wedding 2x2 slices, or fifty party 2x3 slices.
HTH
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