I would start with an 11x 15 if you are not comfortable carving yet.
You can always shave cake off, you cannot put it back on.
I started an XBOX 360 cake almost the size I needed it to be, by the time I was done carving and evening it out, and fixing my goofs it was waaaaay smaller than I intended.
Carving cake isn't as hard as it seems.
I freeze my cake before carving, use a sharp serrated knife and for a cake like this I would cut a semi circle that is as tall as you want the bottle to be out of cardboard. Stick that to the side of the cake that is going to be the bottom, and use than as a template for carving a nice domed shape.
Shave off thin layers, like hair, you can't put it back on if you go too short the first cut.
Hope this helps!