It's hard to tell from the photo, but it looks like that is a 1 5/8" screw. To get the length of a screw you measure from the washer head surface where it seats, to the end of the threaded portion of the screw. Which I would think a standard 1 1/2" screw would be fine.
Mike
This is super easy, like anything else don't rush. Take your time cutting the old foam off the speaker. I have an old set of Cerwin Vegas, that I recently re-foamed, they look and sound like new now.
Mike