I keep getting varied answers. What are the typical dimensions for half a sheet cake?

Yes, you will continue to get different answers
Match the cake size to the boards and boxes sold. In doing that you come up with 12x18. Many use 12x16 instead and that's fine too

Industry standard is 12x18.
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

Thanks!