I'm doing the fender thinking for a hard tail too. On the rear I'm using a Shinko 712, 110/90-18. The Shinko's are a bit wider than other brands of the same size. A 5" fender is what I'll need. I have a '75 Goldwing front fender that I'm thinking about right now. Without changing the length, the widened chrome mud flap area of this fender comes to a good spot over the tire, with plenty of area to support it with a sissy bar. I'm liking the mud flap area of this fender, it reminds me of the '50's cars with wings and fins. So I think I'll be welding and filling over the holes on this fender. Yeah, it looks like a bobber fender with wings.
Front fenders, I think people try to go too small, looks like a sport bike fender, then they decide to go fenderless. I really think I'll decide on a full fender with one or two stays, going for a vintage look and function. 4" wide will probably cover my Shinko 712, 100/90-19 on the front, wish I had the stock fender, I'd be happy with that painted.
Scott