A full sheet is 18"x24"
A half sheet is 12"x18"
A quarter sheet is 9"x12"
The 11"x15" is an odd size, it's a little larger than a 1/3 of the size of the full sheet. I'm not sure how the 11"x15" size came about.
Anyway, most people don't know what the actual sizes of sheets are. People will order a "full" sheet and be expecting anything froma 9"x12" to an 18"x24"!
It's better to refer to the actual dimensions than to use the names.
Use your speed icer and put a thick strip of icing on one side of one cake then put your other half next to it and push them gently together. You should see a little squish(is that even a word???) of icing come up from the "seem"...then your good to go ! Hope this helps...
when you put the two together, how do you you keep them from seperating down the middle? Dumb question?
Not a dumb ? at all. When I started making full cakes which I have since stopped doing (too much hassle), I had a problem with the middle seperating. What I began doing was making sure I used a really strong bottom board to prevent bending & when I ice the cakes I make sure to apply a nice thick amount of buttercream especially down the center. The strong board really makes a difference though. HTH.
Be sure to ask the person how many people they need to feed....then check a serving chart for the needed size.
As someone said....the term "sheet cake" is VERY subjective. Especially if you are doing a one layer cake or two layer.
The number of servings needed is everything!
To me, that's a full sheet.
There is no "to me" to it. We can't just CALL something a half or a full sheet. 
Here's a thread that has the "logic" of why cakes are called the sizes they are called: http://cakecentral.com/cake-decorating-ftopict-628706-.html 
Thanks for the clarifications. I can't seem to find a serving chart that gives me the servings for rectangular cakes though.
The good news about square and rectangle cakes is you don't need a chart ... you can just "Do the Math!" to figure your servings. Here's a thread where I explain how to do that: http://forum.cakecentral.com/cake-decorating-ftopict-653932-math.html
Just one more reason I need a bigger oven then. I can't get anything bigger than my 11x15 pan in my current oven.
Quote by @%username% on %date%
%body%