The key is the following line:
A wizard may cast spells on his familiar even if the spells do not normally affect creatures of the familiar's type (magical beast).
The summoner may cast a spell with a target of “you” on his eidolon (as a spell with a range of touch) instead of on himself. A summoner may cast spells on his eidolon even if the spells normally do not affect creatures of the eidolon’s type (outsider).
So yes, a familiar and/or an eidolon qualifies for Enlarge Person.
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What Kudaku said, but here's a link to the equivalent paragraph under Druid for Animal Companions.
Share Spells (Ex): The druid may cast a spell with a target of “You” on her animal companion (as a spell with a range of touch) instead of on herself. A druid may cast spells on her animal companion even if the spells normally do not affect creatures of the companion's type (animal). Spells cast in this way must come from a class that grants an animal companion. This ability does not allow the animal to share abilities that are not spells, even if they function like spells.
Emphasis mine.
Ah!
There are actually two separate rules here that work separately from each other.
Rule 1: You can cast a spell with a target of "you" on your animal companion/familiar/eidolon instead of yourself.
IE you can cast Mirror Image on your familiar even though Mirror Image has a range of 'personal'.
Rule 2: You can cast a spell on your animal companion/familiar/eidolon even if the spell normally do not affect creatures of the companion's/eidolon's type.
IE you can cast Enlarge Person on your familiar even though Enlarge Person requires a humanoid target.