Generally speaking 7.5" is a good STARTING point for a 60" recurve. The older Bear bows were often braced over 9" and some up to 10". I've never had to go that far, but 8" or a little higher isn't unusual.
As has been stated repeatedly, Bow Hush works great for silencing recurves, especially with Flemish strings. Endless strings do tend to be a bit louder, but I think that also varies with the type of serving used. I also agree that you will have the best luck with an arrow of 10 grains per pound of draw weight or a bit more. Once you hit 12 gpp, you are at the point of diminishing returns from what I have seen.
My 60" Black Bear with a B50 Flemish twist string, moleskin over the string grooves, and single cat whiskers is plenty quiet. I haven't needed to use Bow Hush on it. I mainly use it for bow fishing now, but have hunted deer with it as well.
Keep tinkering, it will get quiet when you hit the right combo.