You cast Enlarge Person on your familiar, thanks to Share Spells.
Nothing stopping you from making that permanent, then, right?
A gnome riding his medium sized familiar goat sounds fun.
There isn't anything in the rules preventing this, but your mileage depends on your GM. If you often run into dispel magic then this might get costly to keep up over time depending on your level. I would discuss this with him/her first.
Aside from making your familiar an easier target to hit, what benefit do you see from doing this? Is the upgrade from their 1d6-4 damage worth the negatives?
Yes, it's RAW legal.
It can even be managed as early as 5th-6th character level, as long as the character can purchase a scroll of permanency (enlarge person) for 3,625 gp and make the DC 10 caster level check.
So then could you cast enlarge person on it, permanency it, then cast animal growth on it?
So then could you cast enlarge person on it, permanency it, then cast animal growth on it?
Nope.
Multiple magical effects that increase size do not stack.
Multiple magical effects that increase size do not stack.
It's legal, but as a GM I would say you're just begging for me to hit you (or your familiar in this case) with dispel magic.
It also makes your familiar a much larger target (both literally and figuratively). As a GM I normally leave familiars alone, assuming they don't meaningfully participate in combat. The moment you start riding one around, have it deliver touch spells, etc I'm going to start trying to kill it.
So...you can't do it, but I recommend caution. And if I were your GM I would advise you against it with the knowledge that it would now become a target.