Não, eles não empilham.
O Perguntas frequentes sobre empilhamento de tamanho afirma:
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).
Domínio improvisado de armas afirma:
Increase the amount of damage dealt by the improvised weapon by one step
Que se enquadram na parte de idioma semelhante da FAQ para aumentos efetivos de tamanho.
Dado que Estilo Shikigami afirma:
While in this style, you deal damage with improvised weapons as if they were one size category larger. For every style feat you have that lists Shikigami Style as a prerequisite, treat the improvised weapon as an additional size category larger, to a maximum of three sizes larger than its actual size.
A cláusula no FAQ sobre como obter o maior aumento efetivo de tamanho se aplica, e você escolheria o maior dos dois, que sempre será do estilo shikigami, pois será o mesmo ou maior que o domínio improvisado de armas.