How will the Warlock's EMP blast affect larger ships like the Idris compared to smaller Constellation and Hornet sized ships?

EMP has no effect on ship hull, only on components. Components that are affected take distortion damage relative to their distance from the source of the EMP blast, so it's far more likely to only partially disrupt a capital ship than a smaller ship. Larger components also have higher tolerance for distortion damage, generally speaking.