Last year they were...
Cake Flavors
Chocolate
Vanilla
Marble
Filling Flavors
Buttercream
White Chocolate Mousse
Dark Chocolate Mousse
Fruit Mousse
Icing
White Buttercream
Chocolate Buttercream
Whipped Cream
However, since then cakes have been turning up with more interesting flavors, like red velvet. Only the pastry chef at the Contemporary can tell you for sure what's available, but for a list of ideas, check out the flavors and fillings available at the Grand Floridian:
http://www.disboards.com/showthread.php?t=1859670
Same as the Contemporary list abovemore details here:
http://disneytravelbabble.com/how-to/order-cake/
This is another question for the pastry chef at your chosen resort because it seems to change all the time. Lately it's been nothing smaller than a 10-inch for BoardWalk Bakery and nothing smaller than an 8-inch for Contemporary, but, the other day someone posted a 6-inch from the Contemporarymay have to do with the size of the decorations requested.
Disney doesn't do yellow cake at any of its bakeries, unfortunately. Will vanilla do? Also, I've never seen foil Mickeys on ganacheI'd guess they don't adhere properlybut don't give up hope til you hear it from the pastry chef.
Hope this helps!