My girlfriend needs a baby shower cake for 30-35 guests. It looks like a 2-layer 12-inch round will serve 40. If I went with a sheet cake (or a partial sheet) and just wanted to make it one layer, would an 11x15 be adequate for that many people?

That would be plenty, I usually make a 1/2 sheet for up to 45 people. Because there is usually a few who don't eat any cake.

I don't know the exact size of the slices you would cut, but in 2x2 slices, and 11x15 cake would yield 35 pieces + 1/2" leftover all around

Ali

I would say an 11x15 feeds 34-36, so that should be perfect. You could also make little indentions in the cake, so whoever cuts it gets enough pieces.
Here is a link to a website that has a cake like this. It is in the Showers/Groom's photo gallery. http://cindascreativecakes.com/Home/homeframe.htm