Size damage modifiers

[BudderHunter20]

I have a human psychic warrior and long story short he can use Karak's Expansion without spending any points and he can use it forever, I cant really explain it as I'm new to psionics and my friends who are better helped me set him up. Anyways I couldn't find anything that would tell me what a heavy spiked shield bash damage would go to from a medium to large size. if anyone can help me that would be great

[Fuzzy-Wuzzy]

You want Table: Tiny and Large Weapon Damage under Equipment in the Core Rulebook.

[Gallant Armor]

1d8 in case you don't want to look at the chart.

[Diego Rossi]

If you are making a character bent on using spiked shied and the ability to enlarge, you should take into account these FAQs:

PRD wrote:

Size increases and effective size increases: How does damage work if I have various effects that change my actual size, my effective size, and my damage dice?

As per the rules on size changes, size changes do not stack, so if you have multiple size changing effects (for instance an effect that increases your size by one step and another that increases your size by two steps), only the largest applies. The same is true of effective size increases (which includes “deal damage as if they were one size category larger than they actually are,” “your damage die type increases by one step,” and similar language). They don’t stack with each other, just take the biggest one. However, you can have one of each and they do work together (for example, enlarge person increasing your actual size to Large and a bashing shield increasing your shield’s effective size by two steps, for a total of 2d6 damage).

and this post by the PDT

Question wrote:

Spiked Bashing Shields deal 1d8 or 2d6?

Pathfinder Design Team Official Rules Response Jun 24, 2016 wrote:

No FAQ Required: As per this FAQ on effective size increases, two effective size increases do not stack. Shield spikes and bashing both grant effective size increases, so they do not stack. The extraneous mention of armor spikes in Ultimate Equipment’s spiked shield entry is in error, and it should be reflected in the next errata.

[Gallant Armor]

Enlarge is an actual size increase while spiked and bashing are effective size increases. You can have one of each. Enlarge and spiked or enlarge and bashing stack, but spiked and bashing would not stack. All of this would also stack with wielding a large shield as an over-sized weapon.

FAQ wrote:

Size increases and effective size increases: How does damage work if I have various effects that change my actual size, my effective size, and my damage dice?
As per the rules on size changes, size changes do not stack, so if you have multiple size changing effects (for instance an effect that increases your size by one step and another that increases your size by two steps), only the largest applies. The same is true of effective size increases (which includes “deal damage as if they were one size category larger than they actually are,” “your damage die type increases by one step,” and similar language). They don’t stack with each other, just take the biggest one. However, you can have one of each and they do work together (for example, enlarge person increasing your actual size to Large and a bashing shield increasing your shield’s effective size by two steps, for a total of 2d6 damage).