It is a fair question, but unfortunately one I can't answer. The catalogue illustration that I have is tiny, and I've enlarged it as much as I can without too much loss of detail.
I would estimate it as 5"- 6" (13cm - 15cm), but don't hold me to it.
This appears to be a foie gras server. They usually came in a set with a serving fork. The spade with a beveled terminal end was used to cut the foie gras, and then the fork was used to lift the slice onto the diner's plate. A full discussion of French hors d'oeuvre services can be found in an article on the website of the Silver Society of Canada.