Vibe is correct up above. With a 100 speed shield, it blocks attacks before it is fully raised. A slow shield like the steel shield blocks AFTER it is fully raised. With a decent amount of shield skill though, anything over 80 and you should be fine. With slower shields, you'll need a bit more shield skill.
Shield skill also increases shield speed and shield durability. I'm not sure whether durability increases armor, hit points or both.
There has been a lot of discussion on how much the force field increases, but it's not THAT much. From what I've found though, with 5 shield skill you should be able to block any arrows that are fired at your front arc. If an archer is trying to hit your feet though, you should still try lowering your shield though.
Just so you know, the forcefield is ONLY for arrows. For melee it still relies on the size of the shield. Unless you're on a horse where your shield seems to magically block the horse too.