I like the idea of hiding behind my shield while being Shouted by a dragon, but it kind of ruins the immersion of it when the dragon's shout goes through it and I end up being chilled or caught on fire. I know that perk giving you 50% resistance to magic on your shield...

Anyway to completely negate all magic damage using a shield (no wards)? So I DON'T catch on fire?

nope.

the shields in Skyrim are not large enough for such a purpose.

you would need a Tower Shield for such a purpose (Not unlike the one in your Avatar), most Shields in Skyrim are a cross between Bucklers or Kite Shields.

as such, the fire simply goes around your shield, as it is not large enough to protect your entire body

But it could not southc me if I had my heat-conducting metal steel boots , gauntlet, and helmet on, right?

I think it caps at 80% resistance. you will take 20% of the damage if you have 80% or more resistance.

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I remember a thread that noted AC protection and magic resistance stack - is that right? So if you armor absorbs 80%, your magic resistance (say the max at 85%) reduces the remaining 20% to 3%. Once you have some regeneration in the mix, you're basically invulnerable, no? My PC has these characteristics and dragon breath on Master difficulty doesn't seem to reduce the health bar at all.

Why don't you use the Spellbreaker?