I was going to guess 5/8" but I think jamie may be closer to the mark with 6/8", you will have to let us know, just don't use calipers to measure it there is (as far as I know) a link to an excellent straight razor ruler in the library.
before you start using your razor you will need a strop, decent soap and a brush.
I agree with the guys who have suggested getting the razor honed, learning shaving and honing concurrently is a path some members have taken but it is the much harder one.