My Large Munsterlander likes to sit on the window sill. It's 6 inches wide
I just don't leave anything on the sills anymore. It's a lot safer, and easier to clean ;)
Dogs are very nosey animals, and unless you are prepared to put a barrier of some sort in the way then you may have to crate, or just put up with it :) I moved a chair under the window so that my dogs could sit on the chair and look out, only now all 3 sit there. :D