Rules about Sizes

[ICEruseseyes]

Hi I understand that there are rules about weapon wieghts, prices and damages however after my last game as a paladin who focus's shield use i wondered since your weapons could be a size category larger like a friends character pregen had a large bastard sword for a melee weapon could I not do the same with my shield and if so how would that affect its ac bonus and ac penalty I've been searching both paizo and the internet and cannot find rules of this. I am currently playing in a local pfs game and know that it is legal to have larger weapons/shield because of legal archetypes and pregens with such things I also know it is legal to wield a "weapon" shield that is 1 size category larger than you since it is one handed. Could i ask someone to tell me where I might find rulings legal in pfs about wielding a larger shield please and thank you.

[Dread Knight]

Shields give you the AC it says it gives no matter the size the only benefit you'd get out of increasing the size of a shield is damage dice for using it as a weapon.

[Bob Bob Bob]

The pregen in question should be Amiri, who uses a Large Bastard Sword. There's a lot of rules language issues around it so I'll skip to the important part which is that she's using a normally one-handed (for her) Large weapon by using it in two hands and taking a -2 penalty to hit. This is the only way she can use it.
Dread Knight is correct that shields don't give any extra AC for size changes, however it would be possible for you to use a Larger shield as a more powerful weapon (with the corresponding penalty to hit for it being too big for you). There's generally no point as a Large Light Shield does damage like a Medium Heavy Shield, but there are a few specific exceptions.

[ICEruseseyes]

Hi I understand that this is the ruled however is it me or does it make no sense that a physically larger objevt doesn't provide better protection in my case the shield would be almost as large as my character. Now could I also ask where you found this ruling so I can quote from it in my local game. Plz and ty.

[Bob Bob Bob]

I'm not sure what ruling you want. If it's that armor and shields don't provide a different bonus based on size, the closest we come is the tables that only give one value for each shield (and no difference for size) and that the "Armor for Unusual Creatures" (which covers larger and smaller creatures) only changes the weight and price of the armor/shields. If you want to claim that a larger Heavy Shield (or whatever it is) should provide more defense than perhaps you could treat it as a Tower Shield, but then it's certainly not whatever it was originally. All of this is from one of the PRD pages on armor.

If it's the ruling on using a shield too large for you as a bigger weapon you just need to look at the weapon table (where Heavy Shield is listed as a one-handed weapon) and the text for inappropriately sized weapons

PRD wrote:

Inappropriately Sized Weapons: A creature can't make optimum use of a weapon that isn't properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. If the creature isn't proficient with the weapon, a –4 nonproficiency penalty also applies.

The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder's size and the size of the creature for which the weapon was designed. For example, a Small creature would wield a Medium one-handed weapon as a two-handed weapon. If a weapon's designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can't wield the weapon at all.

So a Large Heavy Shield would be a two-handed (one step above one-handed) weapon for a Medium creature.