Enlarge person using Leadblade

[Derek Dalton]

Read both spells and from what I can see both work at the same time. Enlarge actually increases you size but Lead Blades simply empowers the weapon to hit as if larger
From the description of both spells you could take for an example a small size creature cat enlarge person making him medium size. He then casts lead blade afterward. So a Halfling normally does a D6 with a longsword now with both spells active now do 2d6.
It does state enlarge can't be cast multiple times. That is the only stipulation. Now Lead Blades doesn't increase the weapns size simply how it hits which suggests while of the same school Transmutation both spells work differently and would stack. Am I wrong in reading it this way?

[Saethori]

They stack.

Basically, you have two forms of size increases allowable. The first is actual size increases, as Enlarge Person accomplishes. The second is virtual size increases that simply cause something to deal damage as if it were larger, as Lead Blades or the Impact enchantment do. For each of these categories, you use whatever benefit is greatest, and they stack with each other.

This can be as mild as two size increases up to Large, as is the case of your medium-sized halfling dealing damage with his Lead Blades-enchanted longsword as if it were Large-sized. Or it can be exaggerated as a Huge-sized human (due to mythic Enlarge Person) dealing damage with his +1 Bashing Heavy Steel Shield as if it were a Colossal-sized weapon.