What would be the best size for a full-width website slider image?

I have a client who needs a slider to be full-width, without any containers or wrappers. I think it's bad practice(you may correct me otherwise), but I couldn't convince her otherwise. I'm concerned about the image sizes. The images are lazy-loaded, but still. One image can take a long time to download if I set it to be too large.

airbnb does it quite well here: http://ift.tt/1DoJOPp

What should the maximum width of the images be?