As I've been looking at ways to maximize save DCs I've seen that eventually you can accrue no more +1DC feats and are forced to resort to debuffing saves. Part of that is intimidate and it seems like a great action for a familiar, but most familiars don't have it as a class skill and also have a low charisma.
Is there a way to build/itemize a familiar for this and overcome the -4 size penalty?
Yeah, those are certainly options, but they're consuming resources each time I need them. I ideally I'm looking for my familiar to be able to intimidate because it will allow me to decide who to intimidated based on who needs to fail a save.
Evolve it for Skilled: Intimidate.
Probably some improved familiars are better at intimidating than others as well.
Faerie Dragons don't have levels in Sorceror nor do they gain Sorceror Class Skills, they CAST AS a Sorceror.
Aberrations and Dragons do have Intimidate as a Class Skill however, per their Creature Type.
If you have an Imp or another Shapechanging Familiar (or use Polymorph spells on them),
the Size penalty doesn't apply when they are in a larger form.
IMHO, you don't even need Intimidate to be uber-maxed out, the DCs just don't scale super high/fast,
ESPECIALLY in a case like this where it is 'free' i.e. not spending your own actions... If it doesn't happen to work one round,
you're not any worse off than everybody else who probably wouldn't have had their Familiar do anything super-useful that round anyways.