If a witch were to cast reduce person on the bat familiar (which has a size of diminutive) using the "Share Spells" feature, would the familiar gain +4 to it's AC / to hit (the effect of reducing a creature's size from diminutive to fine) or would it only be +1 as detailed in the spell (which probably assumes it's only being cast on a small / medium humanoid)?
There's a few problems with that. To start, share spells is only for spells with a target of "you," so reduce person does not require the share spells feature. Second, reduce person only works on humanoids and since it can't be used with share spells there is no way around that limitation. There is a spell called reduce animal that works on animals, but it only works for animals sized small to huge, so diminutive isn't reducible.
If you could somehow magically reduce a diminutive creature, it should gain a +4 to AC and to hit.
You know I had read that to mean that spells with a range of "you" that would not normally affect magical beasts could be used on your familiar, but the way it is written it looks like you may be right. That seems a bit strange though, that you could cast any spell on your familiar.
You know I had read that to mean that spells with a range of "you" that would not normally affect magical beasts could be used on your familiar, but the way it is written it looks like you may be right. That seems a bit strange though, that you could cast any spell on your familiar.
It is fairly controversial. But the note at the bottom of this faq seems to indicate that enlarge person can generally be cast upon an Eidolon via Share Spells (it just notes a discrepancy based on the rule that a synthesist's eidolon must be the same size as the synthesist).
It would give the familiar +4 AC/To hit like any fine creature. I think the text in Reduce Person is just there so you don't have to look up the effect of being small / tiny when it is used on humanoids.