pastrycake,
Do you have "The Cake Bible"? RLBeranbaum? There's lots of info in her book on serving ratio to pan size.
I believe Rose HAS a white chocolate buttercream frosting in her book also. (I find white chocolate frosting persnickety and less stable than plain buttercream. It's low melting point has presented problems for me.)
Re: pan size....Do you mean a full commercial size sheet pan? 18" by 26" I feed around 60 people generously. (More if there are layers) You need a commercial oven to bake in these, and do consider the area of refrigeration necessary (no side by side!). Managing large heavy layers can be unwieldy.
9" x 11" is a standard size sheet pan. As stated, feeds 12.
Save yourself some grief...make cupcakes! Much easier to manage, store and frost..and they're so popular. You can make several varieties, including a vegan option and you'll be the hero. Two excellent cupcake books: Tack | Richardson's "Hello Cupcake" or Martha Stewart's "Cupcakes" and go to town! A good vegan cupcake cookbook is "Vegan Cupcakes Take Over the World".
I like to transport and give cupcakes in these:
Go to: plasticcontainercity.com (click on muffin and cupcake)
Scroll down to item: F2229-89
Dolores