How shields work?

How shields work?

* We know that shields can stop bullets, missiles or a nuke, but can it stop a boarding party moving at slow speed reaching the ship, or alternatively used to block entry to section of a ship or work as containment in reactors?

* How small shields can get? We seen shields used on fighters and personal shields were used for the space drop on Takara, so can they be used in combat (surely couple of bullets is nothing compared to re-entry), on high yield torpedoes to get them through point defense perimeter, or provide extra layer of protection for a fighter bay.

* How creative can we get with shield emitters, can shields be created in angle to deflect projectiles, can two shield fields be layered on on top another?